Fix pom urls (#3499)

Author: yihanzhen

google-cloud-contrib/google-cloud-logging-logback/pom.xml

@@ -8,7 +8,7 @@
   <version>0.55.2-alpha-SNAPSHOT</version><!-- {x-version-update:google-cloud-logging-logback:current} -->
   <packaging>jar</packaging>
   <name>Google Cloud Logging Logback Appender</name>
-  <url>https://github.com/GoogleCloudPlatform/google-cloud-java/tree/master/google-cloud-contrib/google-cloud-logging-logback</url>
+  <url>https://github.com/GoogleCloudPlatform/google-cloud-java/tree/master/google-cloud-clients/google-cloud-contrib/google-cloud-logging-logback</url>
   <description>
     Logback support for Google Cloud Logging
   </description>