RFR: Fix typo in StructuredTaskScope Javadoc

[Anthony Vanelverdinghe]

As the title says.

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Commit messages:
 - Fix typo in StructuredTaskScope Javadoc

Changes: https://git.openjdk.org/loom/pull/195/files
 Webrev: https://webrevs.openjdk.org/?repo=loom&pr=195&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/loom/pull/195.diff
  Fetch: git fetch https://git.openjdk.org/loom pull/195/head:pull/195

PR: https://git.openjdk.org/loom/pull/195