Fix segfault in 1993/plummer

Now it just does nothing if two args are not given to the program. It
seems better to me that it shouldn't crash because of invalid input
unless that's a feature. This is why I've been doing this for other
entries as well. It would be problematic to the program to add a usage
string but I can at least make a check first. Another thought I had was
adding to the Makefile a warning for entries like this but perhaps both
would be the best way to go about it so that users would know that they
had to give certain input for it to work. I don't known but this is a
start anyway.

I also fixed the formatting of the README.md file.

1993/plummer/plummer.c

@@ -1 +1 @@
-char*_,*O;main(S,l)char**l;{*(O=*(l+++S-1)-1)=13,*l[1]=0;for(;;)for(printf(*l),_=O-1;_>=*l&&++*_>(S+*O+S)*S;*_--=(S+*O)*S);}
+char*_,*O;main(S,l)char**l;{if(S>2){*(O=*(l+++S-1)-1)=13,*l[1]=0;for(;;)for(printf(*l),_=O-1;_>=*l&&++*_>(S+*O+S)*S;*_--=(S+*O)*S);}}