hlsdemux: Fix wrong gst-launch command in the description

Fix wrong gst-lauch command in the description.
This patch may help people to get right testing results using the script.

https://bugzilla.gnome.org/show_bug.cgi?id=750143

ext/hls/gsthlsdemux.c

@@ -31,7 +31,7 @@
  * <refsect2>
  * <title>Example launch line</title>
  * |[
- * gst-launch souphttpsrc location=http://devimages.apple.com/iphone/samples/bipbop/gear4/prog_index.m3u8 ! hlsdemux ! decodebin2 ! videoconvert ! videoscale ! autovideosink
+ * gst-launch-1.0 souphttpsrc location=http://devimages.apple.com/iphone/samples/bipbop/gear4/prog_index.m3u8 ! hlsdemux ! decodebin ! videoconvert ! videoscale ! autovideosink
  * ]|
  * </refsect2>
  */