Fix file name of the example hook

Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

contrib/hooks/post-save

@@ -0,0 +1,10 @@
+#!/bin/sh
+#
+# Copy this file into ~/.calcurse/hooks/ and mark it executable to
+# automatically commit any changes to the calcurse data files using Git.
+
+cd "$HOME"/.calcurse/
+git add *
+if ! git diff-index --quiet --cached HEAD; then
+	git commit -m "Automatic commit by the post-save hook"
+fi