Update issue number for async testing on arm64 (#122008)

This one is also presumably fixed.

Cc @dotnet/ilc-contrib

Author: MichalStrehovsky

src/tests/async/Directory.Build.targets

@@ -5,7 +5,7 @@
   </PropertyGroup>
 
   <PropertyGroup>
-    <!-- https://github.com/dotnet/runtime/issues/121878 -->
+    <!-- https://github.com/dotnet/runtime/issues/121871 -->
     <DisableProjectBuild Condition="'$(TestBuildMode)' == 'nativeaot' and '$(TargetArchitecture)' == 'arm64'">true</DisableProjectBuild>
   </PropertyGroup>