{BTree,Hash}Map: add "Entry API" section heading

Author: ada4a

library/alloc/src/collections/btree/map.rs

@@ -135,6 +135,8 @@ pub(super) const MIN_LEN: usize = node::MIN_LEN_AFTER_SPLIT;
 /// ]);
 /// ```
 ///
+/// ## `Entry` API
+///
 /// `BTreeMap` implements an [`Entry API`], which allows for complex
 /// methods of getting, setting, updating and removing keys and their values:
 ///

library/std/src/collections/hash/map.rs

@@ -135,6 +135,8 @@ use crate::ops::Index;
 /// ]);
 /// ```
 ///
+/// ## `Entry` API
+///
 /// `HashMap` implements an [`Entry` API](#method.entry), which allows
 /// for complex methods of getting, setting, updating and removing keys and
 /// their values: