Update rules/Proj0501.md

Co-authored-by: Wesley Baartman <[email protected]>

Author: Corniel

rules/Proj0501.md

@@ -34,7 +34,7 @@ it is possible to specify that these packages are allowed, using the
     <AllowedThirdPartyPackages>
       SonarAnalyzer.Csharp,
       MyCompany.*
-    /AllowedThirdPartyPackages>
+    </AllowedThirdPartyPackages>
   </PropertyGroup>
 
 </Project>