Add step back to player shortcuts list

Author: ku1ik

docs/manual/player/shortcuts.md

@@ -14,7 +14,7 @@ focus:
 * <kbd>Shift</kbd> + <kbd>←</kbd> / <kbd>→</kbd> - rewind / fast-forward by 10%
 * <kbd>[</kbd> / <kbd>]</kbd> - jump to the previous / next [marker](markers.md)
 * <kbd>0</kbd>, <kbd>1</kbd>, <kbd>2</kbd> ... <kbd>9</kbd> - jump to 0%, 10%, 20% ... 90%
-* <kbd>.</kbd> - step through a recording, one frame at a time (when paused)
+* <kbd>,</kbd> / <kbd>.</kbd> - step back / forward, a frame at a time (when paused)
 
 Go ahead and try them out in the player below: