Mention CREATE/DROP STATISTICS in event triggers docs

The new commands are reported by event triggers, but they weren't
documented as such.  Repair.

Author: David Rowley
Discussion: https://postgr.es/m/CAKJS1f-t-NE=AThB3zu1mKhdrm8PCb=++3e7x=Lf343xcrFHxQ@mail.gmail.com

doc/src/sgml/event-trigger.sgml

@@ -503,6 +503,14 @@
         <entry align="center"><literal>-</literal></entry>
         <entry align="center"></entry>
        </row>
+       <row>
+        <entry align="left"><literal>CREATE STATISTICS</literal></entry>
+        <entry align="center"><literal>X</literal></entry>
+        <entry align="center"><literal>X</literal></entry>
+        <entry align="center"><literal>-</literal></entry>
+        <entry align="center"><literal>-</literal></entry>
+        <entry align="center"></entry>
+       </row>
        <row>
         <entry align="left"><literal>CREATE TABLE</literal></entry>
         <entry align="center"><literal>X</literal></entry>
@@ -743,6 +751,14 @@
         <entry align="center"><literal>-</literal></entry>
         <entry align="center"></entry>
        </row>
+       <row>
+        <entry align="left"><literal>DROP STATISTICS</literal></entry>
+        <entry align="center"><literal>X</literal></entry>
+        <entry align="center"><literal>X</literal></entry>
+        <entry align="center"><literal>X</literal></entry>
+        <entry align="center"><literal>-</literal></entry>
+        <entry align="center"></entry>
+       </row>
        <row>
         <entry align="left"><literal>DROP TABLE</literal></entry>
         <entry align="center"><literal>X</literal></entry>