readme: fix typo

PR-URL: https://github.com/joyent/http-parser/pull/207
Reviewed-By: Fedor Indutny <fedor@indutny.com>

README.md

@@ -131,7 +131,7 @@ There are two types of callbacks:
 * notification `typedef int (*http_cb) (http_parser*);`
     Callbacks: on_message_begin, on_headers_complete, on_message_complete.
 * data `typedef int (*http_data_cb) (http_parser*, const char *at, size_t length);`
-    Callbacks: (requests only) on_uri,
+    Callbacks: (requests only) on_url,
                (common) on_header_field, on_header_value, on_body;
 
 Callbacks must return 0 on success. Returning a non-zero value indicates