RFR: 8310915: Typo in aarch64.ad: "envcodings"

[Brian Stafford]

This is a trivial update to a comment that starts with an unnecessary extra slash and contains a typo.

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Commit messages:
 - Minor updates to comment

Changes: https://git.openjdk.org/jdk/pull/20081/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20081&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310915
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20081.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20081/head:pull/20081

PR: https://git.openjdk.org/jdk/pull/20081