Fix grammar in comment of ViewModelBase.cs

[MaxDeVos]

Fix typo in comment

What does the pull request do?

Scope of this PR:

• [x] fix or update to an existing sample
• [ ] add a new sample

What is the current behavior?

Checklist

If this is a new Sample

• [ ] Added a ReadMe-file
• [ ] Updated the landing page
• [ ] Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with a link to your sample in the related documentation page

In any case

• [x] Spell-checking done
• [x] Checked if all hyperlinks work
• [x] Checked if all images are visible

Fixed issues