Add #2943 to the changelog

Author: vy

src/changelog/.2.x.x/3121_instant_format.xml

@@ -4,5 +4,6 @@
        xsi:schemaLocation="https://logging.apache.org/xml/ns https://logging.apache.org/xml/ns/log4j-changelog-0.xsd"
        type="changed">
   <issue id="3121" link="https://github.com/apache/logging-log4j2/pull/3121"/>
+  <issue id="2943" link="https://github.com/apache/logging-log4j2/issues/2943"/>
   <description format="asciidoc">Switch to using Java's `DateTimeFormatter` for date &amp; time formatting of log event instants</description>
 </entry>