Remove some more "in Rust" from titles

src/ch04-03-data-types.md

@@ -1,4 +1,4 @@
-## Data Types in Rust
+## Data Types
 
 Every value in Rust is of a certain *type*, which tells Rust what kind of data
 is being given so it knows how to work with that data. You can usually rely on

src/ch04-04-how-functions-work.md

@@ -1,4 +1,4 @@
-## How Functions Work in Rust
+## How Functions Work
 
 Functions are pervasive in Rust code. We’ve already seen one of the most
 important functions in the language: the `main()` function that’s the entry