buildbot failure in on flink-docs-master

[buildbot]

The Buildbot has detected a failed build on builder flink-docs-master while 
building . Full details are available at:
    https://ci.apache.org/builders/flink-docs-master/builds/1897

Buildbot URL: https://ci.apache.org/

Buildslave for this Build: bb_slave2_ubuntu

Build Reason: forced: by IRC user <LeonardXu> on channel #flink-testing: None
Build Source Stamp: HEAD
Blamelist: 

BUILD FAILED: failed Build docs

Sincerely,
 -The Buildbot