style: put json in quotes

Author: WorkOfStan

README.md

@@ -32,12 +32,12 @@ $rtm = new RTM(APP_KEY, APP_SECRET, PERMISSIONS, FORMAT);
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-| Parameter   | Type   | Default         | Description                                                        |
-| ----------- | ------ | --------------- | ------------------------------------------------------------------ |
-| APP_KEY     | string | none (required) | Your RTM application's public key.                                 |
-| APP_SECRET  | string | none (required) | Your RTM application's secret key.                                 |
-| PERMISSIONS | string | read            | The RTM permission level your application requests from a user.    |
-| FORMAT      | string | json            | The RTM API response format. Value can be either 'json' or 'rest'. |
+| Parameter   | Type   | Default         | Description                                                            |
+| ----------- | ------ | --------------- | ---------------------------------------------------------------------- |
+| APP_KEY     | string | none (required) | Your RTM application's public key.                                     |
+| APP_SECRET  | string | none (required) | Your RTM application's secret key.                                     |
+| PERMISSIONS | string | read            | The RTM permission level your application requests from a user.        |
+| FORMAT      | string | `json`          | The RTM API response format. Value can be either `'json'` or `'rest'`. |
 
 ### Generate Authentication URL