Update nibe_heatpump.markdown

[grapefrukt]

Fixes broken link to M12676EN document

Proposed change

Fixes expired link to the modbus documentation pdf

Type of change

• Spelling, grammar or other readability improvements (current branch).
• Adjusted missing or incorrect information in the current documentation (current branch).
• Added documentation for a new integration I'm adding to Home Assistant (next branch).
  • I've opened up a PR to add logo's and icons in Brands repository.
• Added documentation for a new feature I'm adding to Home Assistant (next branch).
• Removed stale or deprecated documentation.

Additional information

• Link to parent pull request in the codebase:
• Link to parent pull request in the Brands repository:
• This PR fixes or closes issue: fixes #

Checklist

• This PR uses the correct branch, based on one of the following:
  • I made a change to the existing documentation and used the current branch.
  • I made a change that is related to an upcoming version of Home Assistant and used the next branch.
• The documentation follows the Home Assistant documentation standards.

[grapefrukt]

Perhaps it would be better to not rely on NIBE to host this, but this seems to be the current url. I am not knowledgeable enough in the correct procedures for HA to discern where a suitable more permanent location for this file would be.

[c0ffeeca7]

Thank you, @grapefrukt 👍