RFR: 8285440: Typo in Collections.addAll method javadoc

[Johnny Lim]

This PR fixes a typo.

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Commit messages:
 - Fix typo

Changes: https://git.openjdk.java.net/jdk/pull/6942/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6942&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285440
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6942.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6942/head:pull/6942

PR: https://git.openjdk.java.net/jdk/pull/6942