Align DCO wording in '.github/PULL_REQUEST_TEMPLATE.md'

... to that used in 'CONTRIBUTING.md'.

Author: tschwinge

.github/PULL_REQUEST_TEMPLATE.md

@@ -8,7 +8,7 @@ in the git commit message.
 
 Here is a checklist to help you with your PR.
 
-- \[ ] GCC development requires the DCO or copyright assignment. See: https://gcc.gnu.org/contribute.html or https://gcc.gnu.org/dco.html
+- \[ ] GCC development requires copyright assignment or the Developer's Certificate of Origin sign-off, see https://gcc.gnu.org/contribute.html or https://gcc.gnu.org/dco.html
 - \[ ] Read contributing guidlines
 - \[ ] `make check-rust` passes locally
 - \[ ] Run `clang-format`