[Bug][JDK10] Fix duplicated "a" occurrences in docs

[Christoph Dreis]

Hey,

similar to JDK-8190323 the attached patch removes duplicated "a" occurrences
in java.base docs. 

Would be happy if this is sponsored.

I wasn't sure about the capitalization change on EventObject. What's the
rule of thumb in cases where the class does not follow the guidelines for
docs[1]? Sticking to the overall class scheme or aligning everything?

Cheers,
Christoph
[1] http://www.oracle.com/technetwork/articles/java/index-137868.html