Example flag fix - not used this way anymore

[crypt0rr]

As shown below the --nmap-file flag isn't used anymore. This PR fixes only the example text to --file.

$ gowitness nmap --nmap-file out.xml 
Error: unknown flag: --nmap-file
Usage:
  gowitness nmap [flags]

Examples:
# WARNING: These scan all exposed service, like SSH
$ gowitness nmap --nmap-file nmap.xml
$ gowitness nmap --nmap-file nmap.xml --scan-hostnames

# These filter services from the nmap file
$ gowitness nmap --file nmap.xml --service http --service https
$ gowitness nmap --file nmap.xml --service-contains http --service ftp
$ gowitness nmap --file nmap.xml -w http
$ gowitness nmap -f nmap.xml --no-http
$ gowitness nmap -f nmap.xml --no-http --service https --port 8888
$ gowitness nmap -f nmap.xml --no-https -n http -n http-alt
$ gowitness nmap -f nmap.xml --port 80 --port 8080
$ gowitness nmap --nmap-file nmap.xml -s -n http

[leonjza]

🎉