Update error message with too many parameters

src/librustc_ast_lowering/expr.rs

@@ -693,7 +693,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         self.sess,
                         fn_decl_span,
                         E0628,
-                        "too many parameters for generator (expected 0 or 1 parameters)"
+                        "too many parameters for a generator (expected 0 or 1 parameters)"
                     )
                     .emit();
                 }

src/test/ui/generator/too-many-parameters.rs

@@ -1,7 +1,8 @@
 #![feature(generators)]
 
 fn main() {
-    |(), ()| {  //~ error: too many parameters for generator
+    |(), ()| {
+        //~^ error: too many parameters for a generator
         yield;
     };
 }

src/test/ui/generator/too-many-parameters.stderr

@@ -1,4 +1,4 @@
-error[E0628]: too many parameters for generator (expected 0 or 1 parameters)
+error[E0628]: too many parameters for a generator (expected 0 or 1 parameters)
   --> $DIR/too-many-parameters.rs:4:5
    |
 LL |     |(), ()| {