doc: x86/speculation: length of underlines

The lengths of underlines must match the titles to avoid build warnings.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Mark <Gross&lt;mgross@linux.intel.com>
Link: https://lore.kernel.org/r/20200615203645.11545-1-xypron.glpk@gmx.de


Signed-off-by: Jonathan Corbet <corbet@lwn.net>