[IndexTable] Reduce offset to fix overlap (#7433)

### WHY are these changes introduced?

Fixes overlap issue on IndexTable with BulkActions - see Before change
screenshots below

### WHAT is this pull request doing?

<details><summary>Screenshots - Before change</summary>
<img width="1207" alt="Screen Shot 2022-10-18 at 9 16 04 AM"
src="https://user-images.githubusercontent.com/78172179/196473532-4c850944-f97f-471c-bd21-963a6efcb1c4.png">

<img width="594" alt="Screen Shot 2022-10-18 at 9 16 13 AM"
src="https://user-images.githubusercontent.com/78172179/196473530-11e7ed52-bdbe-4856-be3f-ba02fb8725a6.png">

<img width="793" alt="Screen Shot 2022-10-18 at 9 16 32 AM"
src="https://user-images.githubusercontent.com/78172179/196473524-fc0691ba-a929-4dc0-996c-80a819e5d4cf.png">

</details>

<details><summary>Screenshots - After change (current index filtering &
unified index filtering)</summary>

<img width="808" alt="Screen Shot 2022-10-18 at 9 21 46 AM"
src="https://user-images.githubusercontent.com/78172179/196474250-cc58c643-a083-4fe4-bcda-d823292b2d51.png">
<img width="527" alt="Screen Shot 2022-10-18 at 9 21 34 AM"
src="https://user-images.githubusercontent.com/78172179/196474258-222eabfc-259b-4f1b-bb1e-fe9db875f0ab.png">
<img width="781" alt="Screen Shot 2022-10-18 at 9 21 24 AM"
src="https://user-images.githubusercontent.com/78172179/196474262-52bc23c9-390c-4463-81d1-23ae458f94cc.png">

#### With unified_index_filtering beta turned on:
<img width="1213" alt="Screen Shot 2022-10-18 at 9 27 27 AM"
src="https://user-images.githubusercontent.com/78172179/196474782-ef1e9a66-8659-43bb-9829-bfd34c8842d1.png">


</details>


### How to 🎩

Web Spin URL with changes:
https://shop1.shopify.km-indextable.kelly-miller.us.spin.dev/admin/orders?inContextTimeframe=none

1. Visit pages with IndexTable (Orders index, discounts, abandoned
checkouts)
2. Select multiple line items and verify that bulkactions do not overlap
with first row anymore
3. Turn on `unified_index_filtering` beta in your DevUI and verify there
is no overlap

### 🎩 checklist

- [x] Tested on
[mobile](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting.md#cross-browser-testing)
- [x] Tested on [multiple
browsers](https://help.shopify.com/en/manual/shopify-admin/supported-browsers)
- [ ] Tested for
[accessibility](https://github.com/Shopify/polaris/blob/main/documentation/Accessibility%20testing.md)
- [ ] Updated the component's `README.md` with documentation changes
- [ ] [Tophatted
documentation](https://github.com/Shopify/polaris/blob/main/documentation/Tophatting%20documentation.md)
changes in the style guide

Author: kellymiller-shop

.changeset/tame-phones-relax.md

@@ -0,0 +1,5 @@
+---
+'@shopify/polaris': patch
+---
+
+fixed heading offset on IndexTable

polaris-react/src/components/IndexTable/IndexTable.scss

@@ -2,7 +2,7 @@
 
 .IndexTable {
   --pc-index-table-translate-offset: 35px;
-  --pc-index-table-table-header-offset: 52px;
+  --pc-index-table-table-header-offset: 35px;
   --pc-index-table-cell: 1;
   --pc-index-table-sticky-cell: 31;
   --pc-index-table-scroll-bar: 35;