Bump Microsoft.SourceLink.GitHub in /Rx.NET/Source

Bumps [Microsoft.SourceLink.GitHub](https://github.com/dotnet/sourcelink) from 1.0.0-beta2-19401-01 to 1.0.0-beta2-19408-01.
- [Release notes](https://github.com/dotnet/sourcelink/releases)
- [Commits](https://github.com/dotnet/sourcelink/commits)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Rx.NET/Source/Directory.build.props

@@ -24,7 +24,7 @@
   
   
   <ItemGroup>
-    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0-beta2-19401-01" PrivateAssets="All"/>
+    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0-beta2-19408-01" PrivateAssets="All"/>
     <PackageReference Include="Nerdbank.GitVersioning" Version="3.0.24" PrivateAssets="all" />
   </ItemGroup>