docs: replace magnet pattern URLs with correct one

[adigerber]

This is a simple PR that replaces the links to the magnet pattern blog post with the correct one.
The previous link, http://spray.io/blog/2012-12-13-the-magnet-pattern/, is no longer valid - it seems that the spray.io domain was transferred to some other party (a Korean company from the looks of it).
I found the blog post via Google at https://spray.readthedocs.io/en/latest/blog/2012-12-13-the-magnet-pattern.html, so I thought I'd go ahead and submit a fix to the docs so other people don't get confused.

[lightbend-cla-validator]

Hi @adigerber,

Thank you for your contribution! We really value the time you've taken to put this together.

We see that you have signed the Lightbend Contributors License Agreement before, however, the CLA has changed since you last signed it.
Please review the new CLA and sign it before we proceed with reviewing this pull request:

http://www.lightbend.com/contribute/cla

[adigerber]

We see that you have signed the Lightbend Contributors License Agreement before, however, the CLA has changed since you last signed it. Please review the new CLA and sign it before we proceed with reviewing this pull request:

I signed it again I guess

[johanandren]

CLA manually verified 👍

[johanandren]

LGTM, thanks for noticing and fixing @adigerber