Fix a style of the lock button

browser/main/Detail/MarkdownNoteDetail.js

@@ -281,7 +281,7 @@ class MarkdownNoteDetail extends React.Component {
                   onFocus={(e) => this.handleFocus(e)}
                   onMouseDown={(e) => this.handleLockButtonMouseDown(e)}
                 >
-                  <i className={faClassName}/>
+                  <i className={faClassName} styleName='lock-button'/>
                   <span styleName='control-lockButton-tooltip'>
                   {this.state.isLocked ? 'Unlock' : 'Lock'}
                   </span>

browser/main/Detail/MarkdownNoteDetail.styl

@@ -9,6 +9,9 @@
   background-color $ui-noteDetail-backgroundColor
   box-shadow $note-detail-box-shadow
 
+.lock-button
+  padding-bottom 3px
+
 .control-lockButton
   width 34px
   height 34px