Fix pretty print of equals operator

Summary: Equals operator is displayed incorrectly.

Reviewed By: grievejia

Differential Revision: D30219892

fbshipit-source-id: cf7929fa45e8d1493937ffd325dfacb6c21908d2

Author: dyniec

source/ast/expression.ml

@@ -218,7 +218,7 @@ end = struct
       formatter
       "%s"
       (match operator with
-      | Equals -> "="
+      | Equals -> "=="
       | GreaterThan -> ">"
       | GreaterThanOrEquals -> ">="
       | In -> "in"