Add missing word "are"

Author: cakebaker

clippy_lints/src/lines_filter_map_ok.rs

@@ -25,7 +25,7 @@ declare_clippy_lint! {
     ///
     /// ### Known problems
     /// This lint suggests replacing `filter_map()` or `flat_map()` applied to a `Lines`
-    /// instance in all cases. There two cases where the suggestion might not be
+    /// instance in all cases. There are two cases where the suggestion might not be
     /// appropriate or necessary:
     ///
     /// - If the `Lines` instance can never produce any error, or if an error is produced