fix formatting for BUNDLE_PREFER_PATCH variable in man page

[toy]

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

environment variable sticking out as not highlighted

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

[Edouard-chin]

Thanks for the PR! I pushed a commit to correctly build the man pages.

For next time, if there are other man changes you want to make, you need to edit the .ronn files and run rake man:build.

[toy]

@Edouard-chin Thanks!