[Ruby] Fix the cache gems example

and remove trailing spaces

examples.md

@@ -305,9 +305,9 @@ Replace `~/.local/share/renv` with the correct `path` if not using Ubuntu.
 - uses: actions/cache@v1
   with:
     path: vendor/bundle
-    key: ${{ runner.os }}-gems-${{ hashFiles('**/Gemfile.lock') }}
+    key: ${{ runner.os }}-gem-${{ hashFiles('**/Gemfile.lock') }}
     restore-keys: |
-      ${{ runner.os }}-gems-
+      ${{ runner.os }}-gem-
 ```
 When dependencies are installed later in the workflow, we must specify the same path for the bundler.