Fix typo where a table name in a join was singular.

guides/source/active_record_querying.textile

@@ -834,7 +834,7 @@ SELECT categories.* FROM categories
   INNER JOIN posts ON posts.category_id = categories.id
 </sql>
 
-Or, in English: "return a Category object for all categories with posts". Note that you will see duplicate categories if more than one post has the same category. If you want unique categories, you can use Category.joins(:post).select("distinct(categories.id)").
+Or, in English: "return a Category object for all categories with posts". Note that you will see duplicate categories if more than one post has the same category. If you want unique categories, you can use Category.joins(:posts).select("distinct(categories.id)").
 
 h5. Joining Multiple Associations