fix text panel rows limit (#10246)

public/app/plugins/panel/text/editor.html

@@ -17,7 +17,7 @@
 
 <div class="gf-form-inline">
 	<div class="gf-form gf-form--grow">
-		<code-editor content="ctrl.panel.content" rows="20" on-change="ctrl.render()" data-mode="markdown" code-editor-focus="true">
+		<code-editor content="ctrl.panel.content" on-change="ctrl.render()" data-mode="markdown" data-max-lines=20 code-editor-focus="true">
 		</code-editor>
 	</div>
 </div>