Fix invalid JSON in strings.json file

[hpuac]

Motivation:

Within #1488 a small mistake sneaked into the strings.json file.
Since then the JSON file is invalid, a simple comma is missing.

Changes:

This cahnge will add the missing comma to make the JSON file valid again.

Related issue (check one):

• fixes #
• There is no related issue ticket

Checklist (check one):

• I did not change any code (e.g. documentation changes)
• The code change is tested and works locally.

Test-Hardware list (for code changes)

HA Version:
Zigbee2MQTT Version:
TRV Hardware:

New device mappings

• I avoided any changes to other device mappings
• There are no changes in climate.py

[folfy]

Also need to consider if the strings.json is rly used or needed, since we have the translations and it's just a duplicate of the en file. At least should make it a symlink otherwise I'd say. But PR is fine, thanks!