RFR: 8253938: ZGC: Clean up argument names after JDK-8253030

[Per Liden]

JDK-8253030 changed some variable names from timeout_in_millis to timeout_in_micros, but missed to do the same rename
in the header file. This trivial patch renames two arguments in zMark.hpp accordingly.

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Commit messages:
 - 8253938: ZGC: Clean up argument names after JDK-8253030

Changes: https://git.openjdk.java.net/jdk/pull/483/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=483&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253938
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/483.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/483/head:pull/483

PR: https://git.openjdk.java.net/jdk/pull/483