macOS: update brew install instructions
macOS: update brew install instructions

krb5 is available through the brew/dupes tap.

Change-Id: I89a8d28f3fbce2dff526934bd16436e54333cc5c
Reviewed-on: http://gerrit.cloudera.org:8080/5508
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>

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