Update RootNamespaceAttribute.xml (dotnet#724)

Remove weird character ']'

Author: daxianji007

xml/System.Windows.Markup/RootNamespaceAttribute.xml

@@ -96,10 +96,10 @@
         <ReturnType>System.String</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets the string that corresponds to the value of the <see langword="RootNamespace" /> property in a Visual Studio] project file.</summary>
-        <value>The string that corresponds to the value of the <see langword="RootNamespace" /> property in a Visual Studio] project file.</value>
+        <summary>Gets the string that corresponds to the value of the <see langword="RootNamespace" /> property in a Visual Studio project file.</summary>
+        <value>The string that corresponds to the value of the <see langword="RootNamespace" /> property in a Visual Studio project file.</value>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>