docs: Escape unescaped minus sign in asterisk.8 manpage.

ASTERISK-23768 #close
Reported by: Jeremy Lainé
Patches:
  escape_manpage_hyphen.patch uploaded by Jeremy Lainé (License #6561)
........

Merged revisions 423915 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 423916 from http://svn.asterisk.org/svn/asterisk/branches/11
........

Merged revisions 423917 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/13@423918 65c4cc65-6c06-0410-ace0-fbb531ad65f3

doc/asterisk.8

@@ -98,7 +98,7 @@ option also implies \*(T<\fB\-f\fR\*(T> (no forking). However when
 connecting to an existing Asterisk instance (\*(T<\fB\-r\fR\*(T> or
 \*(T<\fB\-R\fR\*(T>), it may only increase the debug level.
 .TP 
--e \fImemory\fR
+\-e \fImemory\fR
 Limit the generation of new channels when the amount of free memory 
 has decreased to under \fImemory\fR megabytes.
 Equivalent to \*(T<\fBminmemfree = \fR\*(T>\fImemory\fR in