Update docs/recipes-images.rst

Add missing f-string.

Co-authored-by: Copilot <[email protected]>

Author: knotapun

docs/recipes-images.rst

@@ -525,8 +525,8 @@ This script creates an approximate image of it as a PNG, by going down to one-pi
     t1 = time.perf_counter()
     pm.save("sierpinski-punch.png")
     t2 = time.perf_counter()
-    print ("{round(t1-t0,3)} sec to create / fill the pixmap")
-    print ("{round(t2-t1,3)} sec to save the image")
+    print (f"{round(t1-t0,3)} sec to create / fill the pixmap")
+    print (f"{round(t2-t1,3)} sec to save the image")
 
 The result should look something like this: