Properly escape particular URL displayed inside the admin page

Inside the Admin Panel, an internal URL was not being properly escaped. This patch uses the esc_url() function to properly escape the URL.

This vulnerability affects the following application versions:

• Contact Form 7 4.4
• Contact Form 7 4.4.1
• Contact Form 7 4.4.2
• Contact Form 7 4.5
• Contact Form 7 4.5.1
• Contact Form 7 4.6
• Contact Form 7 4.6.1
• Contact Form 7 4.7
• Contact Form 7 4.8
• Contact Form 7 4.8.1
• Contact Form 7 4.9
• Contact Form 7 4.9.1
• Contact Form 7 4.9.2
• Contact Form 7 5.0
• Contact Form 7 5.0.1
• Contact Form 7 5.0.2
• Contact Form 7 5.0.3