Update PR template to suggests users to run ./sh/check.sh

Author: emilk

.github/pull_request_template.md

@@ -6,11 +6,10 @@ Please read the "Making a PR" section of [`CONTRIBUTING.md`](https://github.com/
 * Unless this is a trivial change, add a line to the relevant `CHANGELOG.md` under "Unreleased".
 * If it is a non-trivial addition, consider adding a demo for it to `egui_demo_lib`.
 * Remember to run `cargo fmt` and `cargo clippy`.
-* Open the PR as a draft until you have self-reviewed it and it is green.
+* Open the PR as a draft until you have self-reviewed it and run `./sh/check.sh`.
 * When you have addressed a PR comment, mark it as resolved.
 
 Please be patient! I will review you PR, but my time is limited!
 -->
 
 Closes <https://github.com/emilk/egui/issues/THE_RELEVANT_ISSUE>.
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