pinpoint: fix bitrotted reference to sample.txt in README

pinpoint/README

@@ -1,6 +1,6 @@
 pinpoint
 
 pinpoint a simple presentation tool that hopes to encourage small amounts of
-text and images instead of death by bullet points. sample.txt contains a
+text and images instead of death by bullet points. introduction.pin contains a
 demonstration of most features. Look at it in a text editor and run it with
 pinpoint name-of-file.