docs - fix UUID data type size
The UUID data type is actually 16 bytes (128 bits) in size as
described in `src/include/utils/uuid.h` and in Postgres documentation
(https://www.postgresql.org/docs/9.4/datatype-uuid.html). Most likely
it was confused for its 32 digit representation.
Reported by Evgeny Ermakov on OSS Greenplum Slack.
<row>
<entry colname="col1">uuid</entry>
<entry colname="col2"/>
- <entry colname="col3">32 bytes</entry>
+ <entry colname="col3">16 bytes</entry>
<entry colname="col4"/>
<entry colname="col5">Universally Unique Identifiers according to RFC 4122, ISO/IEC
9834-8:2005</entry>
