Use bash code fences for install guide [ci skip]

Using `shell` results in a somewhat different color formatting.
All other command examples set the language to `bash` instead.

Author: p8

guides/source/install_ruby_on_rails.md

@@ -29,7 +29,7 @@ For macOS, you'll need Xcode Command Line Tools and Homebrew to install dependen
 
 Open Terminal and run the following commands:
 
-```shell
+```bash
 # Install Xcode Command Line Tools
 $ xcode-select --install