src: fix Systemtap node_gc_stop probe

The process("node").mark("gc__stop") is actually
process("node").mark("gc__done"). Use the proper name so that a
developer can use SystemTap to determine the duration of garbage
collection.

PR-URL: https://github.com/nodejs/node/pull/20152


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>