Docs: Fix databricks_grants example for metastore

[donatasm]

Changes

Add required metastore argument for databricks_grants in metastore docs example.

Current example would return an error:

Error: Missing required argument
  with databricks_grants.catalog_staging,
  on [catalogs.tf](http://catalogs.tf/) line 24, in resource "databricks_grants" "catalog_staging":
  24: resource "databricks_grants" "catalog_staging" {
"model": one of
`catalog,external_location,foreign_connection,function,metastore,model,schema,share,storage_credential,table,volume`
must be specified

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c87b9fe) 83.76% compared to head (3d3d8d8) 83.79%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3239      +/-   ##
==========================================
+ Coverage   83.76%   83.79%   +0.02%     
==========================================
  Files         170      170              
  Lines       15177    15177              
==========================================
+ Hits        12713    12717       +4     
+ Misses       1718     1716       -2     
+ Partials      746      744       -2     

see 2 files with indirect coverage changes