Clarify algorithmic complexity of ImmutableList<T>.Builder.ToImmutable

As the number of mutations approaches 0, the complexity is actually approaching O(log n).

Fixes dotnet/runtime#14713

Author: marcusturewicz

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

@@ -2349,7 +2349,7 @@ This member is an explicit interface member implementation. It can be used only
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- This method is an O(n) operation and approaches O(1) time as the number of actual mutations to the set since the last call to this method approaches 0.  
+ This method is an O(n) operation and approaches O(log n) time as the number of actual mutations to the set since the last call to this method approaches 0.  
   
  ]]></format>
         </remarks>