docs: add parameter with brackets around varbit()

Reported-by: scott.ure@caseware.com

Discussion: https://postgr.es/m/152074343671.1853.18284519607571497106@wrigleys.postgresql.org

Author: Euler Taveira

Backpatch-through: 10

Author: bmomjian

doc/src/sgml/datatype.sgml

@@ -60,7 +60,7 @@
 
       <row>
        <entry><type>bit varying [ (<replaceable>n</replaceable>) ]</type></entry>
-       <entry><type>varbit</type></entry>
+       <entry><type>varbit [ (<replaceable>n</replaceable>) ]</type></entry>
        <entry>variable-length bit string</entry>
       </row>