RFR: JDK-8272119: Typo in JDK documentation (a -> an)

[Viktor Klang]

a -> an

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Commit messages:
 - Fixing a small typo in Collectors.summingInt

Changes: https://git.openjdk.org/jdk/pull/13353/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13353&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8272119
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/13353.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13353/head:pull/13353

PR: https://git.openjdk.org/jdk/pull/13353