RFR: 8325309: Amend "Listeners and Threads" in AWTThreadIssues.html

[Alexey Ivanov]

Replace _“effect”_ with _“affect”_¹ because the meaning in the phrase “…the changes only _effect_ subsequent notification” is to “have an effect on.”

Add a comma to separate a conditional clause from the main one.

Use `<p>` instead of `<br>`: the example should be either in-line or in a separate paragraph.

Amend grammar: “a key _listeners_ is added.” Here, the usage of the indefinite article and the singular form of _“to be”_ implies the subject is singular.

¹ For reference, see [‘Affect’ vs. ‘Effect’](https://www.merriam-webster.com/grammar/affect-vs-effect-usage-difference).

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Commit messages:
 - Amend Listeners section

Changes: https://git.openjdk.org/jdk/pull/17725/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17725&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325309
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/17725.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17725/head:pull/17725

PR: https://git.openjdk.org/jdk/pull/17725