Fix WinUI3 source code link

[dotMorten]

Description

Updated the link to point to the stable release source code.
The WinUI3 source code link for some time now has pointed to a stale winui3/main branch which doesn't seem to get the source code merged into it. Updated the link to point to the stable release source code so it's easier for users to find and less confusing and avoiding having to browse through the 161 branches trying to find the right one.

Motivation and Context

The link in the readme says it points to source code, but points to an empty branch that hasn't been updated ever since the source code was released.

How Has This Been Tested?

I clicked the link. It worked. It takes you to source code now ;-)

Screenshots (if appropriate):

[Jay-o-Way]

• Do not use "click here" as link text. It's bad for search engine optimization and doesn't adequately describe the target.

  https://learn.microsoft.com/contribute/content/how-to-write-links#link-text

• Why not use the most basic link, like https://github.com/microsoft/microsoft-ui-xaml/tree/main ? Your new link needs to be updated over time. This requires more time that could be spent on something else; and if people forget to update, this link is going to be old

[dotMorten]

@Jay-o-Way while I think your click here link issue is valid it’s out of scope of this. I’m just fixing the link that has been broken for months. I’ll let the team make a rewrite of the readme when they feel like it