runtime(doc): non-portable sed regex in Makefile for pi_netrw.txt rule

runtime(doc): non-portable sed regex in Makefile for pi_netrw.txt rule

Previously it was using '\0' in sed which is non-portable and does not
work in macOS. Fix this by using the '$' (end-of-line) regex atom (which
needs to be doubled in the Makefile) to append at the end instead. An
alternative would have been to use '&' which is the more portable
version of '\0'.

closes: #16996

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>