Update potentially obsolete Sphinx docs build command referenced in documentation

[lunkwill42]

The NAV 5.11 release required the Debian package build script to be slightly altered to still be able to build the Sphinx docs properly.

Specifically, the following two documentation entries may need an update:

https://github.com/Uninett/nav/blob/0c9d015a003cdd13d9117ea9654cfb9d8edf4dac/doc/howto/generic-install-from-source.rst?plain=1#L188

https://github.com/Uninett/nav/blob/a711862409961a092bd6e5cec662bfacd8cfc170/doc/howto/manual-install-on-debian.rst?plain=1#L62