doc: add missing closing parenthesis

Following the style to the nearby links to LICENSE-APACHE
and LICENSE_MIT, the link to LICENSE-UNICODE was placed
after an opening parenthesis, but the closing parenthesis was
missing.

See also #535

PR #536

Author: ScottAbbey

README.md

@@ -235,4 +235,4 @@ at your option.
 
 The data in `regex-syntax/src/unicode_tables/` is licensed under the Unicode
 License Agreement
-([LICENSE-UNICODE](http://www.unicode.org/copyright.html#License).
+([LICENSE-UNICODE](http://www.unicode.org/copyright.html#License)).