wasmtime-py: Don't use "main.rs" for a wasm example so cargo doesn't test it (#464)

cargo test will test examples that have a "main.rs", but in this case,
the example requires manual compilation as a wasm module, and doesn't
build as a standalone program. Rename it to "demo.rs".

misc/wasmtime-py/examples/import/run.py

@@ -1,4 +1,4 @@
 import wasmtime
-import main
+import demo
 
-main.test()
+demo.test()