docs: userspace-api: media: fdl-appendix.rst: replace some characters

The conversion tools used during DocBook/LaTeX/html/Markdown->ReST
conversion and some cut-and-pasted text contain some characters that
aren't easily reachable on standard keyboards and/or could cause
troubles when parsed by the documentation build system.

Replace the occurences of the following characters:

	- U+201c ('“'): LEFT DOUBLE QUOTATION MARK
	- U+201d ('”'): RIGHT DOUBLE QUOTATION MARK

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>