Update MSBuild property to use one from linked doc

Author: vritant24

docs/test/configure-unit-tests-by-using-a-dot-runsettings-file.md

@@ -55,7 +55,7 @@ There are three ways of specifying a run settings file in Visual Studio 2019 ver
     ```xml
     <Project Sdk="Microsoft.NET.Sdk">
       <PropertyGroup>
-        <RunSettingsFilePath>$(SolutionDir)\example.runsettings</RunSettingsFilePath>
+        <RunSettingsFilePath>$(MSBuildProjectDirectory)\example.runsettings</RunSettingsFilePath>
       </PropertyGroup>
       ...
     </Project>