Docs: Reverted "english" to "en".

docs/index.html

@@ -30,7 +30,7 @@
 					<input placeholder="" type="text" id="filterInput" autocorrect="off" autocapitalize="off" spellcheck="false" />
 					<div id="exitSearchButton"></div>
 					<select id="language">
-						<option value="en">english</option>
+						<option value="en">en</option>
 						<option value="zh">中文</option>
 					</select>
 				</div>