Small documentation improvements

[Martin H.]

Hi,

I've pulled my hair out because I wasn't able to build any package. 
Turns out the following line at "Running Guix Before It Is Installed" 
(https://www.gnu.org/software/guix/manual/html_node/Running-Guix-Before-It-Is-Installed.html#Running-Guix-Before-It-Is-Installed):

$ sudo ./pre-inst-env guix-daemon --build-users-group=guixbuild

should rather be

$ sudo -E ./pre-inst-env guix-daemon --build-users-group=guixbuild

Otherwise the daemon won't have the GUILE_LOAD_PATH set correctly. At 
least when running this from a user checkout of Guix on a GuixSD system.

Best
Martin