fix(docs, tools): switch to main in PR template (freeCodeCamp#40882)

Changes `master` to `main` in the template to reflect our branch rename.

Signed-off-by: nhcarrigan <[email protected]>

Author: Nicholas Carrigan (he/him)

.github/PULL_REQUEST_TEMPLATE.md

@@ -4,7 +4,7 @@ Checklist:
 
 - [ ] I have read [freeCodeCamp's contribution guidelines](https://contribute.freecodecamp.org).
 - [ ] My pull request has a descriptive title (not a vague title like `Update index.md`)
-- [ ] My pull request targets the `master` branch of freeCodeCamp.
+- [ ] My pull request targets the `main` branch of freeCodeCamp.
 - [ ] All the files I changed are in the same world language, for example: only English changes, or only Chinese changes, etc.
 
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