doc: Add PROPERTY GRAPH to the access privilege tables

Hi hackers,

Since SQL/PGQ was committed, the SELECT privilege can be granted on a
property graph, but property graphs are missing from the two summary
tables in the "Privileges" section of ddl.sgml.

The description of the SELECT privilege in that section already
mentions property graphs, and GRANT lists PROPERTY GRAPH as a
grantable object, yet the reference tables that follow do not:
- The privilege abbreviations table does not list PROPERTY GRAPH among
the object types the SELECT privilege applies to.
- The "Summary of Access Privileges" table has no PROPERTY GRAPH row.

The attached patch adds the missing entries.

--
Best regards,
Shinya Kato
NTT OSS Center