typo

.github/PULL_REQUEST_TEMPLATE.md

 Make sure these boxes are checked before submitting your pull request:
 
 - [ ] I added a new regtest or modified an existing regtest to validate my changes.
-- [ ] I verified that all regtests are passed succesfully on Travis.
+- [ ] I verified that all regtests are passed successfully on Travis.
 - [ ] I agree to transfer the copyright of the code I have written to the PLUMED developers.