Update renamed API method

See b957d0b8580bafb35fbb808ef1d7acf424f73895

plugin/speakernotes/notes.html

@@ -130,7 +130,7 @@
 				currentSlide.contentWindow.Reveal.nextFragment();
 			}
 			else if (data.fragment === 'previous') {
-				currentSlide.contentWindow.Reveal.previousFragment();
+				currentSlide.contentWindow.Reveal.prevFragment();
 			}
 		});
 		</script>