books/porters-handbook: Fix mistake in vuln.xml description

The example for vuln.xml contains the line
<range><ge>1.6</ge><lt>1.9</lt></range> which is explained as describing
the interval 1.6 <= x <= 1.9, but is 1.6 <= x < 1.9 instead as is
suggested by:

• the <ge> and <lt> tags;
• the output of pkg audit, which for the example would contain the line

1.6: < 1.9.

Reviewed by: mat, crees
Approved by: mat, bcr (docs), gerald (mentor)
Differential Revision: https://reviews.freebsd.org/D24381