Fix typo in Guard.ThrowHelper.ThrowArgumentNullExceptionForIsNotNull

[Benna96]

Super minor typo fix

Closes #862

Removes an extra parenthesis from the message of the exception thrown by Guard.ThrowHelper.ThrowArgumentNullExceptionForIsNotNull.

PR Checklist

• [x] Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• [x] Based off latest main branch of toolkit
• [x] PR doesn't include merge commits (always rebase on top of our main, if needed)
• [ ] Tested code with current supported SDKs
• [ ] Tests for the changes have been added (for bug fixes / features) (if applicable)
• [x] Contains NO breaking changes
• [x] Code follows all style conventions

[Benna96]

@dotnet-policy-service agree

[Benna96]

Is the CLA check supposed to take this long? Did I do something wrong? I just needed to comment with one of the options it gave, right? Been 2 hours. Guess I'll try again.

[Benna96]

@dotnet-policy-service agree