Add missing --prefer-local to Synopsis in bundle-install.1.ronn

[olleolleolle]

What was the end-user or developer problem that led to this PR?

I read about the option, and went to search the web to learn more, and first the 1.12 bundler version was the SEO leader, and I had to navigate to 2.x via the logo link - and then I searched for "install", and found the section about the option.

But, it was not mentioned in the Synopsis section, which would have confirmed its existence earlier.

What is your fix for the problem, implemented in this PR?

I added this to the .ronn file, but I am not currently near a computer, so I have not run any generate docs tasks.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

[martinemde]

Just need to run rake man:build.

[olleolleolle]

@martinemde Indeed, yeah, perfect. Now that's there, and we are green.

[martinemde]

Thanks for catching that and fixing!