rubcops/provided_by_macos: fix wording for core formulae

Library/Homebrew/rubocops/uses_from_macos.rb

@@ -59,9 +59,16 @@ module RuboCop
             return if PROVIDED_BY_MACOS_FORMULAE.include? @formula_name
             return if tap_style_exception? :provided_by_macos_formulae
 
-            problem "Formulae that are `keg_only :provided_by_macos` should be added to "\
+            message = if formula_tap == "homebrew-core"
+              "Formulae in homebrew/core that are `keg_only :provided_by_macos` should be "\
+              "added to the `PROVIDED_BY_MACOS_FORMULAE` list (in the Homebrew/brew repo)"
+            else
+              "Formulae that are `keg_only :provided_by_macos` should be added to "\
               "`style_exceptions/provided_by_macos_formulae.json`"
             end
+
+            problem message
+          end
         end
       end
 

Library/Homebrew/test/rubocops/provided_by_macos_spec.rb

@@ -33,6 +33,18 @@ describe RuboCop::Cop::FormulaAudit::ProvidedByMacos do
     RUBY
   end
 
+  it "fails for homebrew-core formulae not in provided_by_macos_formulae list" do
+    expect_offense(<<~RUBY, "/homebrew-core/")
+      class Baz < Formula
+        url "https://brew.sh/baz-1.0.tgz"
+        homepage "https://brew.sh"
+
+        keg_only :provided_by_macos
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^ Formulae in homebrew/core that are `keg_only :provided_by_macos` should be added to the `PROVIDED_BY_MACOS_FORMULAE` list (in the Homebrew/brew repo)
+      end
+    RUBY
+  end
+
   it "succeeds for formulae in provided_by_macos_formulae list" do
     setup_style_exceptions