Fix comment about complexity ImmutableArray`1.ToBuilder method (#4273)

The operation is O(n) instead of O(1), because it copies all the elements to the builder.

Author: joost-morsink

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

@@ -3728,7 +3728,7 @@ This member is an explicit interface member implementation. It can be used only
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- This is an O(1) operation and results in only a single (small) memory allocation. The mutable list that is returned is not thread-safe.  
+ This is an O(n) operation and copies all the elements of the immutable array into the Builder. The mutable list that is returned is not thread-safe.  
   
  ]]></format>
         </remarks>