Fixed rendering of a header in the book

Author: alex

book/src/nocopy.md

@@ -6,7 +6,7 @@ Clone)]` to a translated type definition will compile, it still shouldn't do
 that for reasons it can't know. In these cases, the `nocopy` annotation can be
 used to prevent bindgen to autoderive the `Copy` and `Clone` traits for a type.
 
-###Library
+### Library
 
 * [`bindgen::Builder::no_copy`](https://docs.rs/bindgen/0.23.1/bindgen/struct.Builder.html#method.no_copy)