Update generator guide [ci skip]

[yhirano55]

Summary

• The result of bin/rails generate generator initializer is different between actual and guide. I've added lines for test unit to logs.
• The generated codes are using single quote not double quote. So I've replace them.
• Use https instead of git.

I wish to make the guide better little by little than yesterday. Thanks.

[rails-bot]

r? @javan

(@rails-bot has picked a reviewer for you, use r? to override)

[vipulnsward]

Thanks, I had these changes myself, dint get time to push them.