chore: update java-docfx-doclet version (googleapis#1457)

Author: eaball35

pom.xml

@@ -719,7 +719,7 @@
             <configuration>
               <doclet>com.microsoft.doclet.DocFxDoclet</doclet>
               <useStandardDocletOptions>false</useStandardDocletOptions>
-              <docletPath>${env.KOKORO_GFILE_DIR}/java-docfx-doclet-1.2.0.jar</docletPath>
+              <docletPath>${env.KOKORO_GFILE_DIR}/java-docfx-doclet-1.2.1.jar</docletPath>
               <additionalOptions>
                 -outputpath ${project.build.directory}/docfx-yml
                 -projectname ${artifactId}