docs: driver-api/thermal/intel_dptf: Use copyright symbol

Using a substitution pattern of "|copy|" without including
isonum.txt causes a doc build warning.

Using the symbol "©" itself is a better choice for those
who read .rst sources.

Reported by: Randy Dunlap <rdunlap@infradead.org>

Fixes: 16c02447 ("Documentation: thermal: DPTF Documentation")
Suggested-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: linux-pm@vger.kernel.org
Cc: linux-doc@vger.kernel.org
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>