RFR: 8272739: Misformatted error message in EventHandlerCreator

[Zhengyu Gu]

Please review this trivial patch to add a missing space in error message.

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Commit messages:
 - v0

Changes: https://git.openjdk.java.net/jdk/pull/5193/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5193&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272739
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5193.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5193/head:pull/5193

PR: https://git.openjdk.java.net/jdk/pull/5193