chore: update java docfx doclet (googleapis#735)

Author: eaball35

pom.xml

@@ -496,7 +496,7 @@
             <configuration>
               <doclet>com.microsoft.doclet.DocFxDoclet</doclet>
               <useStandardDocletOptions>false</useStandardDocletOptions>
-              <docletPath>${env.KOKORO_GFILE_DIR}/java-docfx-doclet-1.1.1.jar</docletPath>
+              <docletPath>${env.KOKORO_GFILE_DIR}/java-docfx-doclet-1.2.0.jar</docletPath>
               <additionalOptions>-outputpath ${project.build.directory}/docfx-yml -projectname ${artifactId}</additionalOptions>
               <doclint>none</doclint>
               <show>protected</show>