docs: fix syntax error in README snippet

Author: itsabdelrahman

README.md

@@ -190,7 +190,7 @@ const connections = {
 };
 
 const mongooseOpts = { // options for mongoose 4.11.3 and above
-  promiseLibrary = Promise;
+  promiseLibrary: Promise;
   autoReconnect: true,
   reconnectTries: Number.MAX_VALUE,
   reconnectInterval: 1000,