fix typo in dynamic.md documentation

Summary: #accept2ship

Reviewed By: Orvid

Differential Revision: D3486732

fbshipit-source-id: 45bfe1daa1dbd1e427fcd18e71e6b9eeb6d6b2b7

folly/docs/Dynamic.md

@@ -120,7 +120,7 @@ which returns an iterator compatible with `items()`:
     // pos->first is "hello"
     // pos->second is "world"
 
-    auto pos = obj.find("no_such_key);
+    auto pos = obj.find("no_such_key");
     // pos == obj.items().end()
 ```