doc: emphasize build support for MySQL 5.7

doc/source/news.rst

@@ -21,7 +21,7 @@ Improvements
 * Supported to specify options by table comment when primary key is using hash by ``PRIMARY KEY (...) USING HASH``.
   [GitHub#46] [Patch by Naoya Murakami]
 * Supported index column flags by index comment. Use ``FULLTEXT INDEX (...) COMMENT 'index_flags "COLUMN_INDEX"'`` for example. [GitHub#47] [Patch by Naoya Murakami]
-* Supported MySQL 5.7.
+* Supported to build with MySQL 5.7.
 * [rpm][centos] Supported Percona Server 5.6.x on CentOS 6/7. [Tested on CentOS 6 by Yoshino]
 * Supported ``utf8_unicode_520_ci`` and ``utf8mb4_unicode_520_ci``. Note that it requires ``groonga-normalizer-mysql`` 1.0.9.