Fix typo (#410)

Author: hiramatsutaku

README.md

@@ -2234,7 +2234,7 @@ const promise = mutate(variables, {
     - `idle` initial status prior to the mutation function executing.
     - `loading` if the mutation is currently executing.
     - `error` if the last mutation attempt resulted in an error.
-    - `success' if the last mutation attempt was successful.
+    - `success` if the last mutation attempt was successful.
 - `data: undefined | Any`
   - Defaults to `undefined`
   - The last successfully resolved data for the query.