Remove surrounding / from file filter

This placeholder is confusing since you're not actually supposed to
surround the regular expression with /.

Originally reported at <https://phabricator.wikimedia.org/T188220>.

Signed-off-by: Kunal Mehta <legoktm@member.fsf.org>

Author: legoktm

ui/assets/js/hound.js

@@ -489,7 +489,7 @@ var SearchBar = React.createClass({
               <div className="field-input">
                 <input type="text"
                     id="files"
-                    placeholder="/regexp/"
+                    placeholder="regexp"
                     ref="files"
                     onKeyDown={this.filesGotKeydown}
                     onFocus={this.filesGotFocus} />