Replace "an" with "a" in the docs

doc/05-repositories.md

@@ -103,7 +103,7 @@ It may include any of the other fields specified in the [schema](04-schema.md).
 
 #### notify-batch
 
-The `notify-batch` field allows you to specify an URL that will be called
+The `notify-batch` field allows you to specify a URL that will be called
 every time a user installs a package. The URL can be either an absolute path
 (that will use the same domain as the repository) or a fully qualified URL.