Update release instructions with branch name change

RELEASE.md

@@ -6,7 +6,7 @@ Then go add this version to the `tf-init` script in the monorepo.
 ```
 $ git remote add upstream git@github.com:cyrilgdn/terraform-provider-postgresql.git
 $ git fetch --all
-$ git merge --no-ff upstream/master  # resolve conflicts, favoring our previous changes
+$ git merge --no-ff upstream/main  # resolve conflicts, favoring our previous changes
 $ git push origin HEAD
 $ git tag v1.20.0-seq
 $ git push origin v1.20.0-seq