Change contributing guide to use safer force push

This commit changes the contributing guide in rails to suggest using git
with force-with-lease over typical force pushing. In practice, most
rails contributors won't ever encounter a situation where updating their
local fork could result in lost changes as a result of a force push.

That being said, git is a complex tool and arcane flags like
force-with-lease are indeed safer, and by promoting it in rails, there's
a chance more people will discover it and use it in other contexts
outside of rails. In just the same way that herd immunity works by most
people being vaccinated, proliferating knowledge of force-with-lease
should help nudge people towards using safer git commands in general.

[ci skip]

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