Update project url on nuget.org (#145)

Author: angelazhangmsft

rnwinrt/nuget/Microsoft.ReactNative.WinRT.nuspec

@@ -12,7 +12,7 @@
     <tags>RN WinRT RNW rnwinrt WinMD xlang</tags>
     <copyright>© Microsoft Corporation. All rights reserved.</copyright>
     <license type="file">LICENSE</license>
-    <projectUrl>https://github.com/microsoft/react-native-windows/tree/main/</projectUrl>
+    <projectUrl>https://github.com/microsoft/react-native-winrt</projectUrl>
   </metadata>
   <files>
     <file src="LICENSE"/>
@@ -25,4 +25,4 @@
     <file src="$reactnative_files$\WinRTTurboModule.targets" target="build\native" />
     <file src="$reactnative_files$\WinRTTurboModule.props" target="build\native" />
   </files>
-</package>
+</package>