Change a comment that was confusing to some users

Prior to this change, the comment directed users to "change EXECUTABLE"
which was the name of the variable whose value was supposed to be
changed. Users often changed the variable name instead of its value.

Makefile

@@ -23,7 +23,7 @@
 # Change IDENTIFIER to match the project identifier given in the project spec.
 IDENTIFIER  = EEC50281EEC50281EEC50281EEC50281EEC50281
 
-# Change EXECUTABLE to match the command name given in the project spec.
+# Change 'executable' to match the command name given in the project spec.
 EXECUTABLE  = executable
 
 # The following line looks for a project's main() in files named project*.cpp,