doc: double escape special characters (\f) in gen-man.sh

sh and, consequently, dash interpret some escaped characters as control
sequences, so an echo '\f' outputs a form feed. To avoid this problem
in manpage generation, we have to escape "\f" sequences.

doc/gen-man.sh

@@ -59,8 +59,8 @@ gem_in=$(
     # First expression replaces all [Text like this] with bold text.
     # Second expression replaces text like *This* or _this_ with italic text.
     sed -E \
-      -e 's#\[([^]]*)\]#\\fB\1\\fR#g' \
-      -e 's#(^|[.,!? ]+)[*_]([^*_ ]+[^*_]+[^*_ ]+)[*_]($|[.,!? ])#\1\\fI\2\\fR\3#g'
+      -e 's#\[([^]]*)\]#\\\\fB\1\\\\fR#g' \
+      -e 's#(^|[.,!? ]+)[*_]([^*_ ]+[^*_]+[^*_ ]+)[*_]($|[.,!? ])#\1\\\\fI\2\\\\fR\3#g'
 )
 
 # Convert gemtext to man format