erb.1: add missing option in detailed description

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60305 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
2017-10-21 16:01:48 +00:00 · 2017-10-21 16:01:48 +00:00 · 6caa91ede2
--- a/man/erb.1
+++ b/man/erb.1
@ -77,6 +77,8 @@ And leading whitespaces are removed if the erb directive starts with
 .Li "<%-" .
 .Pp
 .El
+.It Fl r
+Load a library
 .Pp
 .It Fl U
 can be one of