fix: Fix some mistakes in arithmetic generics docs (#5999)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*



## Additional Context



## Documentation\*

Check one:
- [ ] No documentation needed.
- [x] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

docs/docs/noir/concepts/generics.md

@@ -197,7 +197,7 @@ impl<T, U, let N: u32, let M: u32> Serialize<N + M> for (T, U)
 
 fn main() {
     let data = (1, [2, 3, 4]);
-    assert(data.serialize().len(), 4);
+    assert_eq(data.serialize().len(), 4);
 }
 ```
 

test_programs/compile_failure/arithmetic_generics_intermediate_underflow/src/main.nr

@@ -15,7 +15,7 @@ fn seems_fine<let N: u32>(array: [Field; N]) -> [Field; N] {
 
 fn pop<let N: u32>(array: [Field; N]) -> [Field; N - 1] {
     let mut result: [Field; N - 1] = std::mem::zeroed();
-    for i in 0..N {
+    for i in 0..N - 1 {
         result[i] = array[i];
     }
     result

test_programs/compile_failure/arithmetic_generics_underflow/src/main.nr

@@ -1,7 +1,7 @@
 // docs:start:underflow-example
 fn pop<let N: u32>(array: [Field; N]) -> [Field; N - 1] {
     let mut result: [Field; N - 1] = std::mem::zeroed();
-    for i in 0..N {
+    for i in 0..N - 1 {
         result[i] = array[i];
     }
     result