Remove superfluous `the` from `custom_attribute` feature gate message

src/libsyntax/feature_gate.rs

@@ -399,7 +399,7 @@ fn check_attribute(&self, attr: &ast::Attribute) {
         } else {
             self.gate_feature("custom_attribute", attr.span,
                        &format!("The attribute `{}` is currently \
-                                unknown to the the compiler and \
+                                unknown to the compiler and \
                                 may have meaning \
                                 added to it in the future",
                                 name));