Capitalize testing heading in PR template

[arunsathiya]

Summary

Minor thing: Capitalize testing heading in PR template.

Testing

N/A

Special notes

N/A

Requirements

• [x] I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've ran deno task test after making the changes.

[codecov[bot]]

Codecov Report

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

Comparison is base (b160668) 97.98% compared to head (8b7d87f) 98.07%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #247 +/- ## ========================================== + Coverage 97.98% 98.07% +0.08% ========================================== Files 58 58 Lines 2182 2280 +98 Branches 137 137 ========================================== + Hits 2138 2236 +98 Misses 42 42 Partials 2 2 ```

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