Make sentence clearer (#12786)

I agree with @ChristopherBiscardi this sentence needs a bit more :-)

docs/docs/creating-a-transformer-plugin.md

@@ -272,7 +272,7 @@ Check out the [full source code](https://github.com/gatsbyjs/gatsby/blob/master/
 
 Sometimes transforming properties costs time and resources. In order to avoid recreating these properties at each run, you can profit from the global cache mechanism Gatsby provides.
 
-Cache keys should at least contain the contentDigest of the concerned. For example, the `gatsby-transformer-remark` uses the following cache key for the html node:
+Cache keys should at least contain the contentDigest of the concerned node. For example, the `gatsby-transformer-remark` uses the following cache key for the html node:
 
 ```javascript:title=extend-node-type.js
 const htmlCacheKey = node =>