fix unbalanced comment (#515)

Author: betseg

posts/2020-01-30-Rust-1.41.0.md

@@ -112,7 +112,7 @@ use `Box<T>`, for some specific `T`, while using `T*` in C for the
 corresponding function. As an example, on the C side you may have:
 
 ```c
-// C header */
+// C header
 
 // Returns ownership to the caller.
 struct Foo* foo_new(void);