mpegize: fix double % in output

since there is no format output here double percents are not needed.

mpegize.py

@@ -452,7 +452,7 @@ class Mpegize:
                 if ext=="grw":
                     return
                 print("Phase 1: jpegize", file=sys.stderr)
-                print("Message: 0%%")
+                print("Message: 0%")
                 sys.stdout.flush()
                 m.prepare()
                 m.jpegize()