doc: Ensure intermediate path creation with mkdir

doc: Ensure intermediate path creation with mkdir

The mkdir command in the Installation from Source Short Version docs
didn't use the -p intermediate path creation parameter which likely
would cause the command to fail.  At the time of writing, -p wasn't
universally available but it can now be relied upon existing. The -p
parameter is defined by POSIX, at least since posix-2004.

Reported-by: Daniel Westermann <daniel.westermann@dbi-services.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/ZR0P278MB0920263E7F2D546A33E50079D20E9@ZR0P278MB0920.CHEP278.PROD.OUTLOOK.COM