RFR (JAXP) 8139585: Typo: "APIi" instead of "API"

[huizhe wang]

Hi,

Replaced the texts with the titles of the documents they are referring to.

JBS: https://bugs.openjdk.java.net/browse/JDK-8139585
Webrev: http://cr.openjdk.java.net/~joehw/jdk9/8139585/webrev/

Thanks,
Joe