Fix grammar of the accepted feature warning.

Author: huonw

src/librustc/front/feature_gate.rs

@@ -403,7 +403,7 @@ pub fn check_crate(sess: &Session, krate: &ast::Crate) {
                             sess.span_err(mi.span, "feature has been removed");
                         }
                         Some(&(_, Accepted)) => {
-                            sess.span_warn(mi.span, "feature has added to rust, \
+                            sess.span_warn(mi.span, "feature has been added to Rust, \
                                                      directive not necessary");
                         }
                         None => {

src/test/compile-fail/gated-bad-feature.rs

@@ -20,4 +20,4 @@
 #![feature = "foo"] //~ ERROR: malformed feature
 
 #![feature(test_removed_feature)] //~ ERROR: feature has been removed
-#![feature(test_accepted_feature)] //~ WARNING: feature has added
+#![feature(test_accepted_feature)] //~ WARNING: feature has been added