doc: standardize on `make -j8`

Most of the docs recommend `make -j8`. The one exception is the pull
request template which recommends `make -j4`. This changes the pull
request template so that it is in line with the other docs.

PR-URL: https://github.com/nodejs/node/pull/8678


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>