docs: fix grammatical error (#2080)

source/api/helper.md

@@ -42,7 +42,7 @@ hexo.extend.helper.register('lorem', function(path) {
 });
 ```
 
-### How do I use a registered helper in another extension (e.g. Filter, Injector, etc)?
+### How do I use a registered helper in another extension (e.g. Filter, Injector)?
 
 `hexo.extend.helper.get` will return the helper function, but it needs to have hexo as its context, so:
 

source/api/rendering.md

@@ -70,7 +70,7 @@ If you are not using a [tag plugin](/docs/tag-plugins) and want to use `{{ }}` o
 
 ``` js
 // following example only applies to '.md' file extension
-// you may need to cover other extensions, e.g. '.markdown', '.mkd', etc
+// you may need to cover other extensions, e.g. '.markdown', '.mkd'
 const renderer = hexo.render.renderer.get('md')
 if (renderer) {
   renderer.disableNunjucks = true