docs: Update creating-custom-controls.md

[Xiaoy312]

GitHub Issue (If applicable): n/a

PR Type

What kind of change does this PR introduce?

• Documentation content changes

What is the current behavior?

defining own ResourceDictionary I stumbled over this detail:
https://platform.uno/docs/articles/guides/creating-custom-controls.html
in the text the generic.xaml is lowercase and in the code sample its uppercase. I read somewhere (might be another uno doc page?) that especially for mobile devices the casing is important detail, because if its uppercase, they would not be able to use it. if thats true, this doc page should be updated to recommend lowercase instead in my opinion

What is the new behavior?

unified the casing of "Themes/Generic.xaml" mentioned in this page.

PR Checklist

Please check if your PR fulfills the following requirements:

• Docs have been added/updated which fit documentation template (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
• Validated PR Screenshots Compare Test Run results.
• Contains NO breaking changes
• Associated with an issue (GitHub or internal) and uses the automatic close keywords.
• Commits must be following the Conventional Commits specification.

Other information

https://discord.com/channels/1182775715242967050/1182775715729526908/1314231281453760613

[unodevops]

🤖 Your Docs stage site is ready! Visit it here: https://unodocsprstaging.z13.web.core.windows.net/pr-19018/index.html