Update the dev to main in the PR template (#12834)

Author: Jinjiang

.github/PULL_REQUEST_TEMPLATE.md

@@ -24,7 +24,7 @@ If yes, please describe the impact and migration path for existing applications:
 
 **The PR fulfills these requirements:**
 
-- [ ] It's submitted to the `dev` branch for v2.x (or to a previous version branch), _not_ the `master` branch
+- [ ] It's submitted to the `main` branch for v2.x (or to a previous version branch)
 - [ ] When resolving a specific issue, it's referenced in the PR's title (e.g. `fix #xxx[,#xxx]`, where "xxx" is the issue number)
 - [ ] All tests are passing: https://github.com/vuejs/vue/blob/dev/.github/CONTRIBUTING.md#development-setup
 - [ ] New/updated tests are included