fix code

Author: RalfJung

reference/src/validity/function-pointers.md

@@ -10,15 +10,15 @@ That makes this code UB:
 
 ```rust
 fn bad() {
-    let x: fn() = unsafe { mem::transmute(0usize) } // This is UB!
+    let x: fn() = unsafe { std::mem::transmute(0usize) }; // This is UB!
 }
 ```
 
 However, any integer value other than NULL is allowed for function pointers:
 
 ```rust
 fn good() {
-    let x: fn() = unsafe { mem::transmute(1usize) } // This is not UB.
+    let x: fn() = unsafe { std::mem::transmute(1usize) }; // This is not UB.
 }
 ```
 

rfcs/0000-unaligned-fn-ptr.md

@@ -42,15 +42,15 @@ When transmuting data to a function pointer, you must make sure it is non-null:
 
 ```rust
 fn bad() {
-    let x: fn() = unsafe { mem::transmute(0usize) } // This is UB!
+    let x: fn() = unsafe { std::mem::transmute(0usize) }; // This is UB!
 }
 ```
 
 Transmuting any non-null value to a function pointer is defined behavior.
 
 ```rust
 fn good() {
-    let x: fn() = unsafe { mem::transmute(1usize) } // This is not UB.
+    let x: fn() = unsafe { std::mem::transmute(1usize) }; // This is not UB.
 }
 ```