docs: fix asciidoctor list-index warning in native-ffm-plan

Local Antora build (the same docker pipeline GitHub Actions runs)
emitted:

  warn: native-ffm-plan.adoc:237: list item index: expected 1, got 22

The line started with "22." after a wrap, which the asciidoctor
parser interpreted as a sibling numbered list item with an out-of-
sequence index. Re-wrap so "22." stays mid-sentence. Site rebuild
now warning-free.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: michalharakal

docs/modules/ROOT/pages/explanation/perf/native-ffm-plan.adoc

@@ -233,8 +233,8 @@ the same numbers as today.
 
 == Risks & open questions
 
-. *JDK 21 preview FFM vs JDK 22 stable.* FFM left preview in Java
-22. The repo currently builds on JDK 21 with `--enable-preview
+. *JDK 21 preview FFM vs JDK 22 stable.* FFM left preview in Java 22.
+The repo currently builds on JDK 21 with `--enable-preview
 --add-modules jdk.incubator.vector`. Recommendation: stay on 21
 preview; flip to 22 in a separate toolchain-bump PR.
 . *`MethodHandle` invocation overhead.* Even with FFM, each native