Fix wording in preloading pinned modules docs (#220)

* Fix wording in preloading pinned modules docs

* Apply suggestion

Author: javierjulio

README.md

@@ -124,7 +124,7 @@ Unpinning and removing "object-assign"
 
 ## Preloading pinned modules
 
-To avoid the waterfall effect where the browser has to load one file after another before it can get to the deepest nested import, importmap-rails uses [modulepreload links](https://developers.google.com/web/updates/2017/12/modulepreload) by default. If you don't want to preload a dependencies, because it'you want to load it on-demand for efficiency, pinned modules can prevent preloading by appending `preload: false` to the pin.
+To avoid the waterfall effect where the browser has to load one file after another before it can get to the deepest nested import, importmap-rails uses [modulepreload links](https://developers.google.com/web/updates/2017/12/modulepreload) by default. If you don't want to preload a dependency, because you want to load it on-demand for efficiency, append `preload: false` to the pin.
 
 Example: