* man/irb.1: typo fix

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@20392 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

man/irb.1

@@ -40,7 +40,7 @@ Suppresses read of ~/.irbrc
 .It Fl m
 Bc mode (load mathn, fraction or matrix are available)
 .Pp
-.It Fl -r Ar load-module
+.It Fl r Ar load-module
 Same as `ruby -r'
 .Pp
 .It Fl I Ar path