RFR: 8275783: G1: fix incorrect region type documentation in HeapRegionType

[Albert Mingkun Yang]

Trivial change of fixing documentation.

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Commit messages:
 - region-type

Changes: https://git.openjdk.java.net/jdk/pull/6079/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6079&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275783
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6079.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6079/head:pull/6079

PR: https://git.openjdk.java.net/jdk/pull/6079