RFR: StructuredTaskScope Javadoc: consistently use the phrase "contained in the task scope"

[Anthony Vanelverdinghe]

The class-level Javadoc of StructuredTaskScope contains the following:

> The phrase "threads contained in the task scope" in method descriptions means threads started in the task scope or descendant scopes.

However, the method-level Javadoc sometimes uses slightly different wordings.
This PR updates the Javadoc to consistently use the phrase "contained in the task scope" (and fixes an unrelated typo).

PS: I'll need help from someone to create a JBS issue & sponsor this PR

-------------

Commit messages:
 - Consistently use "contained in the task scope"
 - Fix typo

Changes: https://git.openjdk.org/loom/pull/194/files
 Webrev: https://webrevs.openjdk.org/?repo=loom&pr=194&range=00
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/loom/pull/194.diff
  Fetch: git fetch https://git.openjdk.org/loom pull/194/head:pull/194

PR: https://git.openjdk.org/loom/pull/194