RFR: 8260421: Shenandoah: Fix conc_mark_roots timing name and indentations

[Zhengyu Gu]

Please review this trivial patch that renames conc_mark_roots timing name and fixes indentations

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - JDK-8260421

Changes: https://git.openjdk.java.net/jdk/pull/2241/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2241&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260421
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2241.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2241/head:pull/2241

PR: https://git.openjdk.java.net/jdk/pull/2241