Changed OptString to OptRegexp

Same as in PR #3186
```
firefart@LinuxMint ~/Coding/metasploit-framework $ ./msfcli auxiliary/spoof/nbns/nbns_response 'REGEX=(' E
[*] Initializing modules...
REGEX => (
[-] Auxiliary failed: Msf::OptionValidateError The following options failed to validate: REGEX.

firefart@LinuxMint ~/Coding/metasploit-framework $ ./msfcli auxiliary/spoof/nbns/nbns_response 'REGEX=()' E
[*] Initializing modules...
REGEX => ()
[*] Auxiliary module execution completed

[-] Auxiliary failed: Errno::EACCES Permission denied - bind(2)
[-] Call stack:
[-]   /home/firefart/Coding/metasploit-framework/modules/auxiliary/spoof/nbns/nbns_response.rb:60:in `bind'
[-]   /home/firefart/Coding/metasploit-framework/modules/auxiliary/spoof/nbns/nbns_response.rb:60:in `run'
msf auxiliary(nbns_response) > exit
```