RFR: JDK-8266227: Fix help text for --mac-signing-keychain

[Andy Herrick]

JDK-8266227: Fix help text for --mac-signing-keychain

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Commit messages:
 - JDK-8266227: Fix help text for --mac-signing-keychain

Changes: https://git.openjdk.java.net/jdk/pull/3819/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3819&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266227
  Stats: 89 lines in 3 files changed: 27 ins; 5 del; 57 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3819.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3819/head:pull/3819

PR: https://git.openjdk.java.net/jdk/pull/3819