docs(javadoc): fix param name in sendJsonPayload javadoc

Closes PR #180

Author: ArturMoczulski

rollbar-android/src/main/java/com/rollbar/android/Rollbar.java

@@ -796,7 +796,7 @@ public void log(final Throwable error, final Map<String, Object> custom, final S
   /**
   * Send payload to Rollbar.
   *
-  * @param payload JSON payload string.
+  * @param json JSON payload string.
   */
   public void sendJsonPayload(String json) {
     rollbar.sendJsonPayload(json);