Use "berks vendor" instead of "berks install"

Berkshelf 3.x has been out of beta for a while and the install
command is no longer supported.  This is a quick update to the
documentation to make the bootstrap commands work.

Change-Id: I777a95d868ff51bc26ca40a314fa86df01beb660
Merge "Use "berks vendor" instead of "berks install""