Update index.md

Typo fix.
Replace any usage of 'ifconfig' with the appropriate commands from 'iproute2' as net-tools (providing ifconfig) is mostly deprecated, unmaintained and partially broken.
Flush the IPs of the NIC before adding a static one.
Merge pull request #83 from eliasp/ifconfig-to-iproute2

Update docs wrt the usage of ifconfig
Merge pull request #76 from jeffallen/patch-1

Update index.md