Update shell.md | clarify the use of the OnPlatform markup extension

[bhavanesh2001]

The ShellContent.Icon property for each page uses special syntax: {OnPlatform ...}

The original line inaccurately described the syntax used for ShellContent.Icon. The updated line correctly identifies OnPlatform as a markup extension and explains its purpose in specifying platform-specific values in XAML.

The ShellContent.Icon property for each page uses OnPlatform markup extension. This markup extension in XAML is used to specify different values for different platforms.

[bhavanesh2001]

#sign-off