Fx link in bundle-platform man page

[chtitux]

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

At the end of the page https://bundler.io/man/bundle-platform.1.html , the link to bundle-lock(1) points to https://bundler.io/man/bundle-lock.1.ronn, which is not correct.

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

Internal links should use .html extension, instead of .ronn.

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

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[deivid-rodriguez]

Thanks so much for this @chtitux!