Fixed typo in index.asciidoc

[Cronocide]

trafficstars

https://nmap.org/[namp] -> https://nmap.org/[nmap]

Thanks for contributing to Logstash! If you haven't already signed our CLA, here's a handy link: https://www.elastic.co/contributor-agreement/

[cla-checker-service[bot]]

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[rwaight]

This PR can probably be closed as the change was fixed in https://github.com/logstash-plugins/logstash-codec-nmap/pull/32