Improve how to redefine sleep in 1993/cmills

Using a Makefile variable one can now do

    make clobber SLEEP=200 alt

to change the time to sleep, rather than the more awkward redefining
CDEFINE.

1993/cmills/README.md

@@ -12,7 +12,7 @@ make alt
 To configure how many microseconds to sleep before updates try:
 
 ```sh
-make clobber CDEFINE="-DZ=200" alt
+make clobber SLEEP=200 alt
 ```
 
 NOTE: this entry requires `X11/Xlib.h` header file and the X11 library to
@@ -32,7 +32,6 @@ STATUS: known bug - please help us fix
 For more detailed information see [1993 cmills in bugs.md](/bugs.md#1993-cmills).
 
 
-
 ## To use:
 
 ```sh