RFR: 8261977: Fix comment for getPrefixed() in canonicalize_md.c

[Alexey Semenyuk]

Follow up for JDK-8235397 fix

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Commit messages:
 - Fix comment for getPrefixed() in canonicalize_md.c

Changes: https://git.openjdk.java.net/jdk/pull/2631/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2631&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261977
  Stats: 5 lines in 1 file changed: 0 ins; 4 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2631.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2631/head:pull/2631

PR: https://git.openjdk.java.net/jdk/pull/2631