build: Fix all the Guix development package references.

* Makefile.am (EXTRA_DIST): Use 'guix.scm' instead of the obsolete file
name 'package.scm'.
* example/posts/baz.skr: Likewise.

example/posts/baz.skr

@@ -20,7 +20,7 @@
  (p [And here's a code snippet of how I build Haunt using GNU Guix:])
 
  (source-code
-  "guix environment -l package.scm
+  "guix environment -l guix.scm
 ./configure
 make")