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

Author: danielgustafsson

doc/src/sgml/installation.sgml

@@ -41,7 +41,7 @@ make
 su
 make install
 adduser postgres
-mkdir /usr/local/pgsql/data
+mkdir -p /usr/local/pgsql/data
 chown postgres /usr/local/pgsql/data
 su - postgres
 /usr/local/pgsql/bin/initdb -D /usr/local/pgsql/data