Update reverse_tcp.rb

As I am using a exploit that does a check on the Server HTTP headers to identify the target I saw an error message that reads like this:

>The target server fingerprint "" does not match "(?-mix:(Jetty|JBoss))", use 'set FingerprintCheck false' to disable this check.

Then, while using a HTTP proxy to analyse the requests I am presented with an error that tells me to set another internal option to override a default behaviour. Although it should be pretty clear to everyone using the metasploit framework, I think it is more convenient if all error messages have the same format/way to present suggestions, in this case, presenting the full command the user needs to introduce in order to carry on with the execution of the exploit.