Fix instructions for building rust modules in python examples

When I ran `rustc +nightly ...` the compiler just looked for a source file
called `+nightly`. I changed these instructions to use rustup + rustc instead.

Author: jjsullivan5196

crates/misc/py/examples/gcd/README.md

@@ -3,7 +3,7 @@
 To build `gcd.wasm` use rustc (nightly) for wasm32 target with debug information:
 
 ```
-rustc +nightly --target=wasm32-unknown-unknown -g gcd.rs --crate-type=cdylib
+rustup run nightly rustc --target=wasm32-unknown-unknown -g gcd.rs --crate-type=cdylib
 ```
 
 # Run example

crates/misc/py/examples/import/README.md

@@ -3,7 +3,7 @@
 To build `demo.wasm` use rustc (nightly) for wasm32 target with debug information:
 
 ```
-rustc +nightly --target=wasm32-unknown-unknown demo.rs --crate-type=cdylib
+rustup run nightly rustc --target=wasm32-unknown-unknown demo.rs --crate-type=cdylib
 ```
 
 # Run example

crates/misc/py/examples/two_modules/README.md

@@ -3,7 +3,7 @@
 To build `one.wasm` use rustc (nightly) for wasm32 target with debug information:
 
 ```
-rustc +nightly --target=wasm32-unknown-unknown one.rs --crate-type=cdylib
+rustup run nightly rustc --target=wasm32-unknown-unknown one.rs --crate-type=cdylib
 ```
 
 To build `two.wasm` use wabt.