Fix missing line comment in drop_ref example

Author: nathan

clippy_lints/src/drop_forget_ref.rs

@@ -17,7 +17,7 @@ use utils::{is_copy, match_def_path, opt_def_id, paths, span_note_and_lint};
 /// ```rust
 /// let mut lock_guard = mutex.lock();
 /// std::mem::drop(&lock_guard) // Should have been drop(lock_guard), mutex
-/// still locked
+/// // still locked
 /// operation_that_requires_mutex_to_be_unlocked();
 /// ```
 declare_clippy_lint! {