pgm: fix misspelling of Luigi Rizzo's name.

Author: guyharris

print-pgm.c

@@ -25,7 +25,7 @@
    an *insufficient* description of said changes.  In particular,
    it doesn't indicate what the packet type code for ACK is.
 
-   Luigi Risso's PGMCC code for FreeBSD, at
+   Luigi Rizzo's PGMCC code for FreeBSD, at
 
       https://web.archive.org/web/20020302084503/http://info.iet.unipi.it/~luigi/pgm-code/