docs/specs/index.rst: Fix minor syntax issues

The docs/specs/index.rst has a couple of minor issues which
we didn't notice because we weren't building the manual:
 * the ToC entry for the new PPC XIVE docs points to
   a nonexistent file
 * the initial comment needs to be marked by '..', not '.',
   or it will appear in the output
 * the title doesn't match the capitialization used by
   the existing interop or devel manuals, and uses
   'full-system emulation' rather than the 'system emulation'
   that the interop manual title uses

Fix these minor issues before we start trying to build the manual.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Message-id: 20190610152444.20859-2-peter.maydell@linaro.org