fix dynamic::operator--(int)

Summary: A typo in its name meant it was missing and that `dynamic::operator-(int)` was defined instead.

Reviewed By: ot, luciang

Differential Revision: D31726709

fbshipit-source-id: 42696c958f674de806dd56c3d89ec07dd60db961

folly/dynamic.h

@@ -292,7 +292,7 @@ struct dynamic {
     auto self = *this;
     return ++*this, self;
   }
-  dynamic operator-(int) {
+  dynamic operator--(int) {
     auto self = *this;
     return --*this, self;
   }