Update README.md

Update the json format of session for file, redis, mysql, cookie, there are errors in these json string, such as after the param `ProviderConfig` and there is a lost of `"` in the line 61 of the `gclifetime`.

@@ -58,7 +58,7 @@ Then in you web app init the global session manager
 
 		func init() {
 			globalSessions, _ = session.NewManager(
-				"cookie", `{"cookieName":"gosessionid","enableSetCookie":false,gclifetime":3600,"ProviderConfig":"{\"cookieName\":\"gosessionid\",\"securityKey\":\"beegocookiehashkey\"}"}`)
+				"cookie", `{"cookieName":"gosessionid","enableSetCookie":false,"gclifetime":3600,"ProviderConfig":"{\"cookieName\":\"gosessionid\",\"securityKey\":\"beegocookiehashkey\"}"}`)
 			go globalSessions.GC()
 		}