Fix Rust bindings example (#415)
Fix Rust bindings example (#415)

`Parser::parse` returns `Option<&Tree>`.  An `unwrap()` or similar is required to get the actual `Tree`.
tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter,tree-sitter/tree-sitter