Oops, should use the right separator

Author: davidwengier

src/Shared/Microsoft.AspNetCore.Razor.Utilities.Shared.Test/PathUtilitiesTests.cs

@@ -235,7 +235,7 @@ public void GetDirectoryName_NetFxInvalid(string path)
         // Path.Combine on net472 throws on invalid path characters, so have to do this manually.
         static string PathCombine(string path1, string path2)
         {
-            return $"""{path1}\{path2}""";
+            return $"""{path1}{Path.DirectorySeparatorChar}{path2}""";
         }
     }