[horde] Missing link for Horde 4 docs/INSTALL on website

[Bruno Costacurta]

Hello,

just for information the link for Horde 4 documentation on Horde website
is simply missing as return 404 from the github.com.

See hereafter for incorrect link.

...
For more detailed installation and configuration instructions, e.g. how to
install Horde from PEAR into a location other than the system default, please
see https://github.com/horde/horde/blob/master/horde/docs/INSTALL.
...

Bye,
Bruno