Fix bad regex on length of "Metasploit" string

It won't match a char because it's a newline. While sticking "m" on the
end of the regex would work, there is zero reason we can't hardcode the
length, since the string is fixed.

irb(main):001:0> /.hi/ =~ "\nhi"
=> nil
irb(main):002:0> /.hi/m =~ "\nhi"
=> 0
irb(main):003:0>

modules/exploits/multi/http/jenkins_metaprogramming.rb

@@ -308,7 +308,7 @@ def constructor_class
     )
 
     # Replace length-prefixed string "Metasploit" with a random one
-    klass.sub(/.Metasploit/, "#{[app.length].pack('C')}#{app}")
+    klass.sub("\x0aMetasploit", "#{[app.length].pack('C')}#{app}")
   end
 
   #