Index: wp-admin/includes/meta-boxes.php
===================================================================
--- wp-admin/includes/meta-boxes.php	(revision 36445)
+++ wp-admin/includes/meta-boxes.php	(working copy)
@@ -574,7 +574,7 @@
 function post_excerpt_meta_box($post) {
 ?>
 <label class="screen-reader-text" for="excerpt"><?php _e('Excerpt') ?></label><textarea rows="1" cols="40" name="excerpt" id="excerpt"><?php echo $post->post_excerpt; // textarea_escaped ?></textarea>
-<p><?php _e('Excerpts are optional hand-crafted summaries of your content that can be used in your theme. <a href="https://codex.wordpress.org/Excerpt" target="_blank">Learn more about manual excerpts.</a>'); ?></p>
+<p><?php _e( 'Excerpts are optional hand-crafted summaries of your content that can be used in your theme.' ); ?> <a href="<?php echo esc_url( __( 'https://codex.wordpress.org/Excerpt' ) ); ?>" target="_blank"><?php _e( 'Learn more about manual excerpts.' ); ?></a></p>
 <?php
 }