Update Ruby version examples in README to include 4.0

Author: fkmy

README.md

@@ -79,7 +79,7 @@ jobs:
     - uses: actions/checkout@v6
     - uses: ruby/setup-ruby@v1
       with:
-        ruby-version: '3.4' # Not needed with a .ruby-version, .tool-versions or mise.toml
+        ruby-version: '4.0' # Not needed with a .ruby-version, .tool-versions or mise.toml
         bundler-cache: true # runs 'bundle install' and caches installed gems automatically
     - run: bundle exec rake
 ```
@@ -98,7 +98,7 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest]
         # Due to https://github.com/actions/runner/issues/849, we have to use quotes for '3.0'
-        ruby: ['2.7', '3.0', '3.1', '3.2', '3.3', '3.4', head, jruby, jruby-head, truffleruby, truffleruby-head]
+        ruby: ['2.7', '3.0', '3.1', '3.2', '3.3', '3.4', '4.0', head, jruby, jruby-head, truffleruby, truffleruby-head]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v6
@@ -127,7 +127,7 @@ jobs:
       - uses: actions/checkout@v6
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: '3.4'
+          ruby-version: '4.0'
           bundler-cache: true # runs 'bundle install' and caches installed gems automatically
       - run: bundle exec rake
 ```