RFR: 8346690: Shenandoah: Fix log message for end of GC usage report

[William Kemper]

At the end of a cycle, the non-generational mode usage report has an errant reference to 'generation':

GC(1) At end of GC: generation used: 835M ...

After this change, the message is:

GC(0) At end of GC: used: 1793K ...


The message is unchanged for the generational mode:

GC(0) At end of Concurrent Global GC: Young generation used: 1544K ...
GC(0) At end of Concurrent Global GC: Old generation used: 0B ...

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Commit messages:
 - Fix usage report log message for non-generational modes

Changes: https://git.openjdk.org/jdk/pull/22839/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22839&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346690
  Stats: 18 lines in 1 file changed: 3 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/22839.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22839/head:pull/22839

PR: https://git.openjdk.org/jdk/pull/22839