calc: fix usage of (substring)

plugins/zyklonb/calc

@@ -89,7 +89,7 @@
   (let ([s-len (string-length s)] [with-len (string-length start)])
     (and (>= s-len with-len)
          (string=? (substring s 0 with-len) start)
-         (substring s with-len))))
+         (substring s with-len s-len))))
 
 ; --- Calculator ---------------------------------------------------------------