modify "contributes.languages.id" in order to apply the grammar to .mmd files correctly

[tomoyukim]

Hello. Thank you for providing really great extension! I found the grammar isn't applied correctly in a .mmd file. It seems to be caused by the capital letter in the language ID. I could confirm it worked fine at my local. I'm glad if this PR helps to improve this package.

[rngtng]

Full support this, find the spec here: https://code.visualstudio.com/docs/languages/identifiers

Looking into other plugins, they align to this too, e.g.

• https://github.com/rubyide/vscode-ruby/blob/main/packages/vscode-ruby/package.json#L65
• https://github.com/AnWeber/vscode-httpyac/blob/main/package.json#L58

[rngtng]

^^ Test the fix locally, can confirm it works :)

[rngtng]

@bpruitt-goddard can you pls have a look at this?

[rngtng]

Thanks a lot