Update release notes link to point to main branch for PR template

Author: fselmo

.github/PULL_REQUEST_TEMPLATE.md

@@ -5,7 +5,7 @@ Related to Issue #
 ### How was it fixed?
 
 ### Todo:
-- [ ] Add entry to the [release notes](https://github.com/ethereum/web3.py/blob/master/newsfragments/README.md)
+- [ ] Add entry to the [release notes](https://github.com/ethereum/web3.py/blob/main/newsfragments/README.md)
 
 #### Cute Animal Picture