Add pdf file to 1987/heckbert

To prevent a possible 404 in the future the file
1987/heckbert/FoldSpindleMutilate.pdf was added as it is part of the
judges' remarks.

1987/heckbert/README.md

@@ -33,11 +33,11 @@ Folding at the wrong length may be equivalent to a "Spindle or Mutilate" operati
 ```
 
 Try not to
-[Fold, Spindle or Mutilate](https://repository.library.brown.edu/studio/item/bdr:788264/PDF/)
+[Fold, Spindle or Mutilate](FoldSpindleMutilate.pdf)
 files. :-)
 
 The judges used this program to fold up very long lines which the
-BSD fold(1) was unable to process. This program has been added to
+BSD `fold(1)` was unable to process. This program has been added to
 the contest tool chest.