docs: fix custom elements grammar: singular/plural (#100)

Author: rmuit

content/4.drupal/20.custom-elements.md

@@ -52,7 +52,7 @@ Besides slot, an element may contain any number of attributes, which are key val
 
 ### Cache Metadata
 
-Custom elements integrate with Drupal's [cache API](https://www.drupal.org/docs/8/api/cache-api/cacheabledependencyinterface-friends), thus allows element to define their caching dependencies via cache tags and their cache context. It implements the `CacheableDependencyInterface`, so it's cache dependencies can be easily added as usual, for example entities:
+Custom elements integrate with Drupal's [cache API](https://www.drupal.org/docs/8/api/cache-api/cacheabledependencyinterface-friends), by allowing to define their caching dependencies via cache tags and their cache context. They implement the `CacheableDependencyInterface`, so their cache dependencies can be easily added as usual, for example entities:
 
 ```php
 $custom_element->addCacheableDependency($entity);