doc: add exact informations to disable apparmor

[kfrapin]

Just adding exact informations in order to disable AppArmor when running Vitess locally.

This will save some time when Vitess newbies (like me) try to disable AppArmor :)

Instructions found on the Google Groups.

If modifications need to be done, just tell me !

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[kfrapin]

I signed it!

[googlebot]

CLAs look good, thanks!

[sougou]

LGTM
Thanks for the update!