Fix summary for ImmutableList<T>.Empty (#8148)

Fixes #8129

Author: mjolka

xml/System.Collections.Immutable/ImmutableList`1.xml

@@ -562,7 +562,7 @@
         <ReturnType>System.Collections.Immutable.ImmutableList&lt;T&gt;</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets an empty set with the default sort comparer.</summary>
+        <summary>Gets an empty immutable list.</summary>
         <remarks>To be added.</remarks>
       </Docs>
     </Member>