Make PR template reference supported architectures (elastic#42919)

This commit changes the GitHub PR template to refer to supported "OS
and architecture" (rather than use OS) since we only accept PRs for
x86_64 (and not Linux ARM, s390, etc)

.github/PULL_REQUEST_TEMPLATE.md

@@ -10,5 +10,5 @@ attention.
 - Have you followed the [contributor guidelines](https://github.com/elastic/elasticsearch/blob/master/CONTRIBUTING.md)?
 - If submitting code, have you built your formula locally prior to submission with `gradle check`?
 - If submitting code, is your pull request against master? Unless there is a good reason otherwise, we prefer pull requests against master and will backport as needed.
-- If submitting code, have you checked that your submission is for an [OS that we support](https://www.elastic.co/support/matrix#show_os)?
+- If submitting code, have you checked that your submission is for an [OS and architecture that we support](https://www.elastic.co/support/matrix#show_os)?
 - If you are submitting this code for a class then read our [policy](https://github.com/elastic/elasticsearch/blob/master/CONTRIBUTING.md#contributing-as-part-of-a-class) for that.