Add one missing and fix two broken links

Author: hahnjo

structure/README.md

@@ -6,3 +6,4 @@ Therefore all tests in this category write a single `Int32` field with type `std
  * [`clusters`](clusters): multiple clusters
  * [`cluster_groups`](cluster_groups): multiple cluster groups
  * [`empty`](empty): empty RNTuple without entries
+ * [`feature_flag`](feature_flag): unknown feature flag

types/user/README.md

@@ -1,4 +1,4 @@
 # User-defined Types
 
- * [`class`]: user-defined class(es)
- * [`enum`]: user-defined enum(s)
+ * [`class`](class): user-defined class(es)
+ * [`enum`](enum): user-defined enum(s)