Update webdav.md

[jrb80]

Included QNAP NAS server in list.

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[laurent22]

hello, you need to sign the cra

[jrb80]

I have read the CLA Document and I hereby sign the CLA

[laurent22]

Step: Spellchecking...
> yarn cspell --no-progress --no-summary --config cspell.json **/*.{ts,tsx,md,mdx}
The following spelling mistakes were found. Please check https://joplinapp.org/help/dev/spellcheck for
information on how to deal with spelling mistakes.

/home/runner/work/joplin/joplin/readme/apps/sync/webdav.md:17:4 - Unknown word (QNAP)
Error: Process completed with exit code 1.