[Docs] Fix sub-heading in start-stop-ilm.asciidoc (#51045)

Removed superfluous `=`.

docs/reference/ilm/start-stop-ilm.asciidoc

@@ -79,7 +79,7 @@ The operating modes of ILM:
 |===
 
 [float]
-=== Stopping ILM=
+=== Stopping ILM
 
 The ILM service can be paused such that no further steps will be executed
 using the <<ilm-stop,Stop API>>.