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:

nav/doc/howto/generic-install-from-source.rst

Line 188 in 0c9d015

python setup.py build_sphinx

nav/doc/howto/manual-install-on-debian.rst

Line 62 in a711862

python setup.py build_sphinx