template: Change architecture value for Arm AArch64

[afaerber]

PR creator: Description

Commit 09d801c updated the template README, referenced from the pull request template, to mandate new files add an architecture prefix of aarch for AArch64.

Change that mandated value to arm.

It seems it was not yet in use for existing DC- files. NVIDIA Jetson deployment would be a candidate for adopting it going forward.

PR creator: Are there any relevant issues/feature requests?

No.

PR reviewer: Checklist for editorial review

Apart from the usual checks, please double-check also the following:

• do any new files fulfill the naming conventions specified in https://github.com/SUSE/doc-modular/blob/main/templates/README.md?
• article title/structure title: does it have the required length and does it use sentence style capitalization?
• revhistory: is it up-to-date and does it follow the guidelines specified in https://documentation.suse.com/style/current/html/style-guide-db/sec-smartdocs.html#sec-revhistory?
• metadata: does it follow the guidelines specified in https://documentation.suse.com/style/current/html/style-guide-db/sec-smartdocs.html#sec-taglist-smartdocs?

[afaerber]

Related: Existing files seem to use x86-64, whereas the README says x86.

[afaerber]

Affects #658 file naming.