File stale path references.

After rearranging the directory layout, some paths in documentation
needed updating.

Fix some typos too.

lib/cretonne/meta/build.py

@@ -1,6 +1,6 @@
 # Second-level build script.
 #
-# This script is run from src/libcretonne/build.rs to generate Rust files.
+# This script is run from lib/cretonne/build.rs to generate Rust files.
 
 from __future__ import absolute_import
 import argparse