Use "screen" instead of "breakpoint"

[pedroborges]

Continuing from PR #24, I used breakpoints because it was used on the What is Tailwind? page. Turns out it's used in many other places. This PR fixes that.

[pedroborges]

By the way, the word breakpoint is used in many instances other than referencing the utility prefixes. You may want to check those out as well.

[mikebronner]

Just a comment for consideration as it relates only to the naming convention (I have no idea what the official naming intent is):
screen implies use for display purposes only, where-as breakpoint is more general and can be applied (in a contextual sense) on any media.
I know that breakpoints are important in many custom web apps that make use of print features like reports, etc.

[adamwathan]

Thanks, this is definitely more consistent 👍🏻

[HellPat]

@mikebronner see #12 and maybe leave your comment there

[mikebronner]

@psren Thanks for the cross-link. :) I see it was already heavily under consideration and decided that @screen can be used in a more liberal sense that just a computer screen. Sounds good to me. :)