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.

layouts/partials/helper/image.html

@@ -8,7 +8,7 @@
     {{ $url := urls.Parse $imageValue }}
 
     {{ if or (eq $url.Scheme "http") (eq $url.Scheme "https") }}
-        <!-- Is a external image -->
+        <!-- Is an external image -->
         {{ $result = merge $result (dict "permalink" $imageValue) }}
     {{ else }}
         {{ $pageResourceImage := .Context.Resources.GetMatch (printf "%s" ($imageValue | safeURL)) }}
@@ -58,4 +58,4 @@
 
 {{ end }}
 
-{{ return $result }}
+{{ return $result }}