Fix typo ActiveSyncManager.java，Syntax error in comments

[brondou666]

What changes are proposed in this pull request?
Syntax error in comments
Modify the comments in the ActiveSyncManager.java file, and change a to an
Why are the changes needed?
Please clarify why the changes are needed. For instance,

Fix typo.
Does this PR introduce any user facing changes?
none

Fixed #623 Alluxio/Community#623

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[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in two weeks if no further activity occurs. Thank you for your contributions.

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in two weeks if no further activity occurs. Thank you for your contributions.

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed in two weeks if no further activity occurs. Thank you for your contributions.