Typo fixes in 2020/yang/README.md

It appears that the entry name is 'Mile' but since it's compiled to be
'prog' the command invocations should refer to ./prog not ./mile. I kept
the other references to Mile as is.

I'm aware that this entry does not seem to work under macOS but it could
be that I've been working on this year too much and I need to finish up
(fortunately I have only one more entry to look at). But it might be
that it's simply not functional under macOS.

2020/yang/README.md

@@ -44,7 +44,7 @@ spoiler](spoiler.html) that shows how this entry was written.
 ## Author's comments:
 ### Synopsis
 
-    echo "Hello world" | ./mile 12345 > encoded.c
+    echo "Hello world" | ./prog 12345 > encoded.c
 
     gcc -DPIN=12345 encoded.c -o ./decoded && ./decoded
 
@@ -59,7 +59,7 @@ Mile is a C code generator.  Run without arguments for the help message below:
 When run with an encoding PIN (first command line argument), Mile will
 read bytes from stdin and generate C code on stdout.  Example:
 
-    echo "Hello, world!" | ./mile 56789 > encoded.c
+    echo "Hello, world!" | ./prog 56789 > encoded.c
 
 This generated code will reproduce the same input bytes on stdout when
 executed, if it was compiled with the same encoding PIN.