Fix grammar in error message

Noticed in rust-lang#22429

Author: steveklabnik

src/librustc/middle/stability.rs

@@ -261,12 +261,12 @@ impl<'a, 'tcx> Checker<'a, 'tcx> {
                 if self.tcx.sess.features.borrow().unmarked_api {
                     self.tcx.sess.span_warn(span, "use of unmarked library feature");
                     self.tcx.sess.span_note(span, "this is either a bug in the library you are \
-                                                   using and a bug in the compiler - please \
+                                                   using or a bug in the compiler - please \
                                                    report it in both places");
                 } else {
                     self.tcx.sess.span_err(span, "use of unmarked library feature");
                     self.tcx.sess.span_note(span, "this is either a bug in the library you are \
-                                                   using and a bug in the compiler - please \
+                                                   using or a bug in the compiler - please \
                                                    report it in both places");
                     self.tcx.sess.span_note(span, "use #![feature(unmarked_api)] in the \
                                                    crate attributes to override this");