docs: add missing disk_wb_rate row in gpperfmon system table reference
<codeph>disk_rb_rate</codeph>
</entry>
<entry>bigint</entry>
+ <entry>Bytes per second for disk read operations.</entry>
+ </row>
+ <row>
+ <entry>
+ <codeph>disk_wb_rate</codeph>
+ </entry>
+ <entry>bigint</entry>
<entry>Bytes per second for disk write operations.</entry>
</row>
<row>
