Docs: Fix DataGridAggregationExample

[ScarletKuro]

Description

Fixes: https://github.com/MudBlazor/MudBlazor/issues/7466

How Has This Been Tested?

Visually

Type of Changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Documentation (fix or improvement to the website or code docs)

Checklist

• [x] The PR is submitted to the correct branch (dev).
• [x] My code follows the code style of this project.
• [ ] I've added relevant tests.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.11%. Comparing base (28bc599) to head (ff6337e). Report is 101 commits behind head on dev.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #8772 +/- ## ========================================== + Coverage 89.82% 90.11% +0.28% ========================================== Files 412 419 +7 Lines 11878 12195 +317 Branches 2364 2401 +37 ========================================== + Hits 10670 10990 +320 + Misses 681 662 -19 - Partials 527 543 +16 ```

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