Update flatpak-builder versions in README

Author: davidmhewitt

README.md

@@ -29,7 +29,7 @@ jobs:
       options: --privileged
     steps:
     - uses: actions/checkout@v2
-    - uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@v3
+    - uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@v4
       with:
         bundle: palette.flatpak
         manifest-path: org.gnome.zbrown.Palette.yml
@@ -111,7 +111,7 @@ jobs:
       options: --privileged
     steps:
     - uses: actions/checkout@v2
-    - uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@v3
+    - uses: bilelmoussaoui/flatpak-github-actions/flatpak-builder@v4
       name: "Build"
       with:
         bundle: palette.flatpak