Use correct scope for Razor-plus grammar

[lildude]

Description

Whilst generating and testing the upcoming v7.1.0 release I noticed that the cshtml/razor change in #4314 introduced the wrong scope for the grammar and this is picked up by the grammar compiler:

- [ ] repository `vendor/grammars/razor-plus` (from https://github.com/austincummings/razor-plus) (2 errors)
    - [ ] Unexpected scope in repository: `text.html.cshtml` declared in `text.html.cshtml` (in `syntaxes/cshtml.tmLanguage.json`) was not listed in grammars.yml
    - [ ] Missing scope in repository: `text.html.razor` is listed in grammars.yml but cannot be found

The correct scope is text.html.cshtml as the error reports and can be confirmed by checking the grammar itself. This PR corrects the scope as using the wrong scope results in no syntax highlighting at all.

This would have been picked up by the script/add-grammar script that should have been used to replace the grammar. I suspect it wasn't, hence this slipped through.

Checklist:

• I am adding new or changing current functionality
  • I have added or updated the tests for the new or changed functionality.

A quick review on this would be ✨ so I can get the release back into testing today.

/cc @worldbeater FYI as the author of #4314

[Alhadis]

We should really have a test which catches this...

[lildude]

We should really have a test which catches this...

Yeah, though we really need to address all of the outstanding issues in #3924 first, or maybe whitelist those outstanding grammars in the mean time.