porters-handbook: Fix some variable expansions

The Porter's Handbook expands the variables DATADIR, DOCSDIR and
EXAMPLESDIR as PREFIX/shared/PORTNAME, PREFIX/shared/doc/PORTNAME and
PREFIX/shared/examples/PORTNAME respectively.

As it can be checked by running "make -V" in any port directory, they
are expanded into PREFIX/shared/PORTNAME, PREFIX/shared/doc/PORTNAME and
PREFIX/shared/examples/PORTNAME instead.

Approved by: doc (dbaio), gerald (mentor)
Differential Revision: https://reviews.freebsd.org/D35022