summary update for interior mutability

README.md

@@ -86,9 +86,9 @@ I am a Canadian who lives in Korea, and I wrote Easy Rust while thinking of how
   - [Lifetimes](#lifetimes)
   - [Interior mutability](#interior-mutability)
     - [Cell](#cell)
-  - [RefCell](#refcell)
-  - [Mutex](#mutex)
-  - [RwLock](#rwlock)
+    - [RefCell](#refcell)
+    - [Mutex](#mutex)
+    - [RwLock](#rwlock)
   - [Cow](#cow)
   - [Type aliases](#type-aliases)
     - [Importing and renaming inside a function](#importing-and-renaming-inside-a-function)