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 <[e-mail]>

Fixes: 16c02447f3e1 ("Documentation: thermal: DPTF Documentation")
Suggested-by: Jonathan Corbet <[e-mail]>
Signed-off-by: Akira Yokosawa <[e-mail]>
Cc: "Rafael J. Wysocki" <[e-mail]>
Cc: Srinivas Pandruvada <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Acked-by: Randy Dunlap <[e-mail]>
Tested-by: Randy Dunlap <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/driver-api/thermal/intel_dptf.rst

···
 Intel(R) Dynamic Platform and Thermal Framework Sysfs Interface
 ===============================================================
 
-:Copyright: |copy| 2022 Intel Corporation
+:Copyright: © 2022 Intel Corporation
 
 :Author: Srinivas Pandruvada <[e-mail]>