Update docblock type for $json parameter

This `$json` parameter passed as a Guzzle `json` request option is intended to be an `array` as referenced in the calling Intercom endpoint classes.

Author: ngstylez

src/IntercomClient.php

@@ -141,7 +141,7 @@ public function setClient($client)
      * Sends POST request to Intercom API.
      *
      * @param  string $endpoint
-     * @param  string $json
+     * @param  array $json
      * @return mixed
      * @throws \GuzzleHttp\Exception\GuzzleException
      */
@@ -164,7 +164,7 @@ public function post($endpoint, $json)
      * Sends PUT request to Intercom API.
      *
      * @param  string $endpoint
-     * @param  string $json
+     * @param  array $json
      * @return mixed
      * @throws \GuzzleHttp\Exception\GuzzleException
      */
@@ -188,7 +188,7 @@ public function put($endpoint, $json)
      * Sends DELETE request to Intercom API.
      *
      * @param  string $endpoint
-     * @param  string $json
+     * @param  array $json
      * @return mixed
      * @throws \GuzzleHttp\Exception\GuzzleException
      */