RFR [9] 8132468: docs: replace <tt> tags (obsolete in html5) for java.io, java.lang, java.math

Hello, Could you please review the following fix: http://cr.openjdk.java.net/~avstepan/8132468/webrev.00/ for https://bugs.openjdk.java.net/browse/JDK-8132468 Just some cleanup for docs (replacing obsolete "<tt></tt>"). Thanks, Alexander