fix(0004): use snake case for pattern variable

aep/general/0004/aep.md.j2

@@ -58,7 +58,7 @@ message UserEvent {
     plural: "user-events"
     // define one or more patterns, e.g. if a resource has more than one parent.
     pattern: "projects/{project_id}/user-events/{user_event_id}"
-    pattern: "folder/{folder_id}/user-events/{user_event_d}"
+    pattern: "folder/{folder_id}/user-events/{user_event_id}"
     pattern: "users/{user_id}/events/{user_event_id}"
   };
 
@@ -91,11 +91,12 @@ element = variable | literal;
 variable = "{", literal, "}";
 ```
 
-Where `literal` matches the regex `[a-z][a-z0-9\-_]*[a-z0-9]`.
+Where `literal` matches the regex `[a-z](?:[a-z0-9\-_]*[a-z0-9])?`.
 
 - Patterns **must** match the possible [paths](/paths) of the resource.
 - Pattern variables (the segments within braces) **must** match the singular of
   the resource whose id is being matched by that value, suffixed with `_id`.
+- Pattern variables (the segments within braces) **must** muse snake case.
 
 #### Pattern uniqueness