Missing link to AppRun file in manual packaging doc

[myxo]

https://github.com/AppImage/docs.appimage.org/blob/master/source/packaging-guide/manual.rst

The AppRun file can be a script or executable. It sets up required environment variables such as $PATH and launches the payload application. You can write your own, but in most cases it is easiest (and most error-proof) to use a precompiled one from this repository.

I assume under "this repository" should be link, otherwise it's confusing.

[andy5995]

@myxo I think the link should be https://github.com/AppImage/AppImageBuild

I'll open a PR to fix that.