Path.xml: Update GetFullPath summary (dotnet#4228)

Changed the order of "relative path" and "fully qualified base path" to match the actual order that is required.

Author: awsr

xml/System.IO/Path.xml

@@ -1349,7 +1349,7 @@ For more information on file path formats on Windows, see [File path formats on
       <Docs>
         <param name="path">A relative path to concatenate to <paramref name="basePath" />.</param>
         <param name="basePath">The beginning of a fully qualified path.</param>
-        <summary>Returns an absolute path from a fully qualified base path and a relative path.</summary>
+        <summary>Returns an absolute path from a relative path and a fully qualified base path.</summary>
         <returns>The absolute path.</returns>
         <remarks>
           <format type="text/markdown"><![CDATA[