Fix cache key getter function name at en/caching.md (vuejs#134)

Author: jbruni

en/caching.md

@@ -69,7 +69,7 @@ export default {
 
 Note that cache-able component **must also define a unique `name` option**. With a unique name, the cache key is thus per-component: you don't need to worry about two components returning the same key.
 
-The key returned from `serverCacheKey` should contain sufficient information to represent the shape of the render result. The above is a good implementation if the render result is solely determined by `props.item.id`. However, if the item with the same id may change over time, or if render result also relies on another prop, then you need to modify your `getCacheKey` implementation to take those other variables into account.
+The key returned from `serverCacheKey` should contain sufficient information to represent the shape of the render result. The above is a good implementation if the render result is solely determined by `props.item.id`. However, if the item with the same id may change over time, or if render result also relies on another prop, then you need to modify your `serverCacheKey` implementation to take those other variables into account.
 
 Returning a constant will cause the component to always be cached, which is good for purely static components.