examples: fix build of mux and resample_audio

The commits eac4324bfb and
cd8211527e renamed the examples, but the
targets were not updated. Hence, the builds are missing -lm.

Signed-off-by: Sebastian Ramacher <sramacher@debian.org>
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit 7de3fcb059)

doc/examples/Makefile.example

@@ -43,8 +43,8 @@ OBJS=$(addsuffix .o,$(EXAMPLES))
 # the following examples make explicit use of the math library
 avcodec:           LDLIBS += -lm
 encode_audio:      LDLIBS += -lm
-muxing:            LDLIBS += -lm
-resampling_audio:  LDLIBS += -lm
+mux:               LDLIBS += -lm
+resample_audio:    LDLIBS += -lm
 
 .phony: all clean-test clean