docs: correct grammar mistakes in CONTRIBUTING.md

[i3anaan]

Various grammar mistakes were present in the contribution guidelines
This commit corrects some of them

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• No code change -> no new tests
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Some grammar issues in CONTRIBUTING.md

Issue Number: N/A

What is the new behavior?

Fewer grammar issues in CONTRIBUTING.md

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

My first contribution of hopefully more to come, any feedback is welcome!

[ngbot]

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     missing required label: "PR target: *"
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