Fix grammar errors in apps.mdx

[glen-84]

Motivation and Context

Improved grammar and consistency.

How Has This Been Tested?

n/a

Breaking Changes

Yes, the ui/notifications/tool-cancelled event is now named ui/notifications/tool-canceled.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

cancelled is spelled canceled in US English.