Verify successful parse in Rust binding doctest
We were verifying that the parse operation succeeds, but not verifying
that the example code was valid.  Now we do!
//! let mut parser = Parser::new();
//! parser.set_language(tree_sitter_python::language()).expect("Error loading Python grammar");
//! let parsed = parser.parse(code, None);
-//! # assert!(parsed.is_some());
+//! # let parsed = parsed.unwrap();
+//! # let root = parsed.root_node();
+//! # assert!(!root.has_error());
//! ```
//!
//! [Language]: https://docs.rs/tree-sitter/*/tree_sitter/struct.Language.html
