fix: update java-docfx-doclet version (#315)

Fixes b/200689560

Author: eaball35

pom.xml

@@ -699,7 +699,7 @@
             <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 ${project.artifactId}</additionalOptions>
               <doclint>none</doclint>
               <show>protected</show>