RFR: 8221537: ZGC: Fix incorrect comment in zNMethod table entry layout

[Per Liden]

This comment in zNMethodTableEntry.hpp

//  * 63-3 NMethod Address (61-bits)

should be

//  * 63-2 NMethod Address (62-bits)


Bug: https://bugs.openjdk.java.net/browse/JDK-8221537
Webrev: http://cr.openjdk.java.net/~pliden/8221537/webrev.0

/Per