Bump crate versions in Rust README

Author: dcreager

bindings/rust/README.md

@@ -8,8 +8,8 @@ way.)
 
 ``` toml
 [dependencies]
-tree-sitter = "0.19"
-tree-sitter-javascript = "0.19"
+tree-sitter = "0.20"
+tree-sitter-javascript = "0.20"
 ```
 
 Typically, you will use the [language][language func] function to add this