doc/muxers: fix reference to @anchor defined in another manual

In particular, fix warnings:
*** Undefined node `quoting_and_escaping' in @ref (in doc/muxers.texi l. 767)
*** Undefined node `quoting_and_escaping' in @ref (in doc/muxers.texi l. 772)

doc/muxers.texi

@@ -764,13 +764,13 @@ to feed the same packets to several muxers directly.
 The slave outputs are specified in the file name given to the muxer,
 separated by '|'. If any of the slave name contains the '|' separator,
 leading or trailing spaces or any special character, it must be
-@ref{quoting_and_escaping, escaped}.
+escaped (see the ``Quoting and escaping'' section in the ffmpeg-utils
+manual).
 
 Options can be specified for each slave by prepending them as a list of
 @var{key}=@var{value} pairs separated by ':', between square brackets. If
 the options values contain a special character or the ':' separator, they
-must be @ref{quoting_and_escaping, escaped}; note that this is a second
-level escaping.
+must be escaped; note that this is a second level escaping.
 
 Example: encode something and both archive it in a WebM file and stream it
 as MPEG-TS over UDP: