Fix typo in `mrblib/range.rb` [ci skip]

mrblib/range.rb

@@ -26,7 +26,7 @@ class Range
       return self
     end
 
-    if val.kind_of?(String) && last.kind_of?(String) # fixnums are special
+    if val.kind_of?(String) && last.kind_of?(String) # strings are special
       if val.respond_to? :upto
         return val.upto(last, exclude_end?, &block)
       else