Improve CStr::from_ptr example in docs

Documentation of `CStr::from_ptr` suggests using `str::from_utf8(slice.to_bytes()).unwrap()`
to obtain a `&str` but `CStr` has `CStr::to_str` that does exactly that.

src/libstd/ffi/c_str.rs

@@ -415,8 +415,7 @@ impl CStr {
     ///
     /// unsafe {
     ///     let slice = CStr::from_ptr(my_string());
-    ///     println!("string returned: {}",
-    ///              str::from_utf8(slice.to_bytes()).unwrap());
+    ///     println!("string returned: {}", slice.to_str().unwrap());
     /// }
     /// # }
     /// ```