Fix xlog docs

Summary: When I was investigating the difference between glog/xlog I encountered this.

Reviewed By: yfeldblum

Differential Revision: D6572117

fbshipit-source-id: 4130deb6cfed7cfb7d2433cdab292481beb2469f

folly/experimental/logging/docs/Comparisons.md

@@ -34,7 +34,7 @@ Other advantages of folly logging over glog:
   diminished functionality on Windows, since it cannot be controlled on a
   per-module basis.
 
-Advantages of glog over folly logging over glog:
+Advantages of glog over folly logging:
 
 * Smaller generated code size.  Due to automatically picking a log category
   name, folly logging's `XLOG()` macros currently result in slightly larger