Minor Typo Fix (#491)

The second `t` in `Comitting` was too silent.

docs/en/mutations.md

@@ -153,7 +153,7 @@ mutations: {
 
 Now imagine we are debugging the app and looking at the devtool's mutation logs. For every mutation logged, the devtool will need to capture a "before" and "after" snapshots of the state. However, the asynchronous callback inside the example mutation above makes that impossible: the callback is not called yet when the mutation is committed, and there's no way for the devtool to know when the callback will actually be called - any state mutation performed in the callback is essentially un-trackable!
 
-### Commiting Mutations in Components
+### Committing Mutations in Components
 
 You can commit mutations in components with `this.$store.commit('xxx')`, or use the `mapMutations` helper which maps component methods to `store.commit` calls (requires root `store` injection):