Accept &str in Parse::parse_sql (#182)

It is more generic to accept a `&str` than a `String` in an API,
and avoids having to convert a string to a `String` when not
needed, avoiding a copy.

src/lib.rs

@@ -28,7 +28,7 @@
 //!            WHERE a > b AND b < 100 \
 //!            ORDER BY a DESC, b";
 //!
-//! let ast = Parser::parse_sql(&dialect, sql.to_string()).unwrap();
+//! let ast = Parser::parse_sql(&dialect, sql).unwrap();
 //!
 //! println!("AST: {:?}", ast);
 //! ```