Followup to 7eaeb6d9: fix typo in schema_design.adoc
Followup to 7eaeb6d9: fix typo in schema_design.adoc

Accidentally wrote 'unix_microtime' instead of 'unixtime_micros' in the
documentation in the previous patch.

I also separated out the 64-bit types for better clarity, since it
wasn't obvious before that 'unixtime_micros' is a 64-bit type.

Change-Id: Ied008e4ae1dea0243cb944470cbad2053bebd3d7
Reviewed-on: http://gerrit.cloudera.org:8080/4354
Tested-by: Kudu Jenkins
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

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