Fix grammar

Author: Manishearth

src/doc/grammar.md

@@ -568,7 +568,7 @@ binop_expr : expr binop expr ;
 The precedence of Rust binary operators is ordered as follows, going from
 strong to weak:
 
-```
+```text
 * / %
 as
 + -

src/doc/trpl/ffi.md

@@ -439,7 +439,7 @@ fn main() {
     unsafe { 
         rl_prompt = prompt.as_ptr();
 
-        println!("{}", rl_prompt);
+        println!("{:?}", rl_prompt);
 
         rl_prompt = ptr::null();
     }