media: hdmi: cec: replace broken link to HDMI specs

Current link died, according to Wayback Machine, back in 2017. And the
website is completely down since 2019. Moreover, there was a custom
cover on that PDF, i.e. it was modified.

According to HDMI licence agreement (LA), HDMI specification and technical
information are supposed to be hosted on www.hdmi.org exclusively, and not
redistributed by third-parties.

Sure, there are still many more or less reliable "mirrors" out there with
a direct download straight from a search engine's page. However, for
example, from FPGA4fun[1] website it was removed "per HDMI LA request".

Unfortunately, the official download page is protected by email CAPTCHA,
but that seems to be the only legit way to obtain a copy.

[1] https://www.fpga4fun.com/HDMI.html

Signed-off-by: ivan tkachenko <me@ratijas.tk>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/driver-api/media/cec-core.rst

@@ -26,7 +26,7 @@ It is documented in the HDMI 1.4 specification with the new 2.0 bits documented
 in the HDMI 2.0 specification. But for most of the features the freely available
 HDMI 1.3a specification is sufficient:
 
-http://www.microprocessor.org/HDMISpecification13a.pdf
+https://www.hdmi.org/spec/index
 
 
 CEC Adapter Interface