Update PR template to refer to 5.3.x

.github/PULL_REQUEST_TEMPLATE.md

@@ -16,5 +16,5 @@
   These must match the expectations made by your contribution. 
   You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example `./bin/generate-samples.sh bin/configs/java*`. 
   For Windows users, please run the script in [Git BASH](https://gitforwindows.org/).
-- [ ] File the PR against the [correct branch](https://github.com/OpenAPITools/openapi-generator/wiki/Git-Branches): `master`, `5.1.x`, `6.0.x`
+- [ ] File the PR against the [correct branch](https://github.com/OpenAPITools/openapi-generator/wiki/Git-Branches): `master`, `5.3.x`, `6.0.x`
 - [ ] If your PR is targeting a particular programming language, @mention the [technical committee](https://github.com/openapitools/openapi-generator/#62---openapi-generator-technical-committee) members, so they are more likely to review the pull request.