www/foswiki: remove redundant escape

does not need to be escaped in this context; bug #229925 will make this

escape as well as the escape of most ordinary characters an error to reduce
friction when some of these ordinary characters are later granted special
GNU-extended behavior.

No functional change, no need to bump PORTREVISION.

PR: 240309
Approved by: koobs (mentor), bapt (mentor)
Approved by: portmgr (maintainer timeout: 4 months)
Differential Revision: https://reviews.freebsd.org/D23698