Fix typo in diagnostic

Author: varkor

src/librustc_typeck/collect.rs

@@ -1155,7 +1155,7 @@ fn generics_of(tcx: TyCtxt<'_>, def_id: DefId) -> &ty::Generics {
                             param.span,
                             &format!(
                                 "defaults for type parameters are only allowed in \
-                                        `struct`, `enum`, `type`, or `trait` definitions."
+                                        `struct`, `enum`, `type`, or `trait` definitions"
                             ),
                         );
                     }

src/test/ui/feature-gates/feature-gate-default_type_parameter_fallback.stderr

@@ -1,4 +1,4 @@
-error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions.
+error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
   --> $DIR/feature-gate-default_type_parameter_fallback.rs:3:8
    |
 LL | fn avg<T=i32>(_: T) {}
@@ -8,7 +8,7 @@ LL | fn avg<T=i32>(_: T) {}
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #36887 <https://github.com/rust-lang/rust/issues/36887>
 
-error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions.
+error: defaults for type parameters are only allowed in `struct`, `enum`, `type`, or `trait` definitions
   --> $DIR/feature-gate-default_type_parameter_fallback.rs:8:6
    |
 LL | impl<T=i32> S<T> {}