RFR: 8266185: Shenandoah: Fix incorrect comment/assertion messages

[linade]

Fix incorrect comment and assertion messages in Shenandoah GC

-------------

Commit messages:
 - 8266185: Shenandoah: Fix incorrect comment/assertion messages

Changes: https://git.openjdk.java.net/jdk/pull/3750/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3750&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266185
  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3750.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3750/head:pull/3750

PR: https://git.openjdk.java.net/jdk/pull/3750