Review: 8157925: Fix typo in X509KeyManager javadoc

[Jamil Nimeh]

Hi folks, can I please get a quick review for a very simple javadoc fix 
in X509KeyManager?

Bug: https://bugs.openjdk.java.net/browse/JDK-8157925
Webrev: http://cr.openjdk.java.net/~jnimeh/reviews/8157925/webrev.01
Thanks!
--Jamil