Fix type signature typo in Javadoc

[wtanaka]

No description provided.

[coveralls]

Coverage decreased (-0.003%) to 70.187% when pulling 9eb8f55 on wtanaka:signaturetypo into c9e55a4 on apache:master.

[coveralls]

Coverage decreased (-29.1%) to 41.081% when pulling 9eb8f55 on wtanaka:signaturetypo into c9e55a4 on apache:master.

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/8809/

Failed Tests: 2

beam_PreCommit_Java_MavenInstall/org.apache.beam:beam-examples-java: 1

• org.apache.beam.examples.WindowedWordCountIT.testWindowedWordCountInStreaming

beam_PreCommit_Java_MavenInstall/org.apache.beam:beam-runners-apex: 1

• org.apache.beam.runners.apex.translation.ParDoTranslatorTest.testAssertionFailure

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[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/8810/
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[iemejia]

LGTM

[iemejia]

Thanks @wtanaka for the contribution.