Documentation: fix speculation.rst chapter

'Mitigating speculation side-channels' should be a chapter rather
than title.

Signed-off-by: Cui Wei <chris.wei.cui@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20250426135609.735-1-chris.wei.cui@gmail.com>

Author: bensenq

Documentation/staging/speculation.rst

@@ -63,7 +63,6 @@ of an out-of-bounds address, while the second call will influence
 microarchitectural state dependent on this value. This may provide an
 arbitrary read primitive.
 
-====================================
 Mitigating speculation side-channels
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