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/README.md

@@ -1,9 +1,9 @@
 # Build example's file
 
-To build `main.wasm` use rustc (nightly) for wasm32 target with debug information:
+To build `demo.wasm` use rustc (nightly) for wasm32 target with debug information:
 
 ```
-rustc +nightly --target=wasm32-unknown-unknown main.rs --crate-type=cdylib
+rustc +nightly --target=wasm32-unknown-unknown demo.rs --crate-type=cdylib
 ```
 
 # Run example

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

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