fix(typo): change 'a ex.*' for 'an ex.*' (#213)

In order to fix this kind of typo, the (not sophisticated) Regular
Expression: `[a-zA-Z] [aA] [aeiouAEIOU]` was ran.

That Regular Expression matches some good written sentences like `a
User`, therefore, those has been omitted.

exampleSite/content/categories/Test/_index.md

@@ -1,9 +1,9 @@
 ---
 title: "Test"
-description: "This is a example category"
+description: "This is an example category"
 slug: "test"
 image: "hutomo-abrianto-l2jk-uxb1BY-unsplash.jpg"
 style:
     background: "#2a9d8f"
     color: "#fff"
----
+---