Fix a readme link

[paaljoachim]

Hey

I am working on setting up a prototype for a more modern, better readability and improved structure of the WordPress handbooks: https://make.wordpress.org/docs/handbook/

Looking at various solutions on how to do this.

I have tried various wiki plugins. Various have their advantages and disadvantages.
I came here after some searching.

I noticed the link below in the readme:

"This plugin works with the p2-breathe theme, available at https://wpcom-themes.svn.automattic.com/p2-breathe"
The link goes to a file list.
I did a search and noticed that the them is retired.

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The wordpress.org theme could really do for a good makeover. Along the way this will likely improve various aspects including the handbooks.