fix typo in Relay Mutations & updates doc

Reviewed By: captbaritone, alunyov

Differential Revision: D48988086

fbshipit-source-id: 4d1feaa437afb194c64b292f4b7d0f5ac11eb1ef

Author: Mingshi Yang

website/docs/tutorial/mutations-updates.md

@@ -372,7 +372,7 @@ function StoryLikeButton({story}) {
 
 ### Step 4 — Modify the Updatable Data
 
-Now `upatableData` is an object representing our existing Story as it exists in the local store. We can read and write the fields listed in our fragment:
+Now `updatableData` is an object representing our existing Story as it exists in the local store. We can read and write the fields listed in our fragment:
 
 ```
 function StoryLikeButton({story}) {

website/versioned_docs/version-v15.0.0/tutorial/mutations-updates.md

@@ -371,7 +371,7 @@ function StoryLikeButton({story}) {
 
 ### Step 4 — Modify the Updatable Data
 
-Now `upatableData` is an object representing our existing Story as it exists in the local store. We can read and write the fields listed in our fragment:
+Now `updatableData` is an object representing our existing Story as it exists in the local store. We can read and write the fields listed in our fragment:
 
 ```
 function StoryLikeButton({story}) {