Change 'display' to 'output' in swaylock docs where necessary

swaylock/main.c

@@ -255,7 +255,7 @@ int main(int argc, char **argv) {
 		"  -s, --scaling                  Scaling mode: stretch, fill, fit, center, tile.\n"
 		"  -t, --tiling                   Same as --scaling=tile.\n"
 		"  -v, --version                  Show the version number and quit.\n"
-		"  -i, --image [<display>:]<path>  Display the given image.\n"
+		"  -i, --image [<output>:]<path>  Display the given image.\n"
 		"  --socket <socket>              Use the specified socket.\n";
 
 	registry = registry_poll();

swaylock/swaylock.1.txt

@@ -26,8 +26,8 @@ Options
 	Turn the screen into the given color instead of white. If an image is in use,
 	also enables color display.
 
-*-i, \--image* [<display>:]<path>::
-	Display the given image, optionally only on the given display.
+*-i, \--image* [<output>:]<path>::
+	Display the given image, optionally only on the given output.
 
 *-s, \--scaling*::
 	Scaling mode for images: stretch, fill, fit, center, or tile.