Docs: Fix command for magerun

[kkrieger85]

Magerun command in README should be fixed:

magerun magescan:scan  ✔  561  13:55:48

[Symfony\Component\Console\Exception\CommandNotFoundException]
Command "magescan:scan" is not defined.

Did you mean one of these?
magescan:scan:unreachable
magescan:scan:catalog
magescan:scan:sitemap
magescan:scan:version
magescan:scan:module
magescan:scan:server
magescan:scan:patch
magescan:scan:all