Fix mathematically incorrect statement in IsProperSupersetOf documentation

Co-authored-by: eiriktsarpalis <[email protected]>

Author: Copilot

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

@@ -679,7 +679,7 @@ An empty set is a proper subset of any other collection. Therefore, this method
 
 ## Remarks
 
-An empty set is a proper superset of any other collection. Therefore, this method returns `true` if the collection represented by the `other` parameter is empty, unless the current set is also empty.
+An empty set is a subset of any other collection. Therefore, this method returns `true` if the collection represented by the `other` parameter is empty, unless the current set is also empty.
 
 [System.Collections.Immutable NuGet package](https://www.nuget.org/packages/System.Collections.Immutable/)