buildbot failure in on asterixdb-master-docs

[buildbot]

The Buildbot has detected a new failure on builder asterixdb-master-docs while 
building . Full details are available at:
    https://ci.apache.org/builders/asterixdb-master-docs/builds/1857

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

Buildslave for this Build: asf945_ubuntu

Build Reason: The Nightly scheduler named 'asterixDocsNightly' triggered this 
build
Build Source Stamp: [branch master] HEAD
Blamelist: 

BUILD FAILED: failed

Sincerely,
 -The Buildbot