Wordpress - My custom made plugin has "a new version available" which links to unrelated plugin

I think you've got a naming conflict there -- assuming that your plugin is linking to this Simple Contact Form -- try changing the name to something like "Wilco's Simple Contact Form".

You'll need to update the plugin folder name and the main plugin file name as well.

Update

As Aniket points out, you might need to force an update check on your site to get rid of the notice. You can do this by going to http://yourwebsite.com/wp-admin/update-core.php?force-c‌​heck=1

As i can see you have named your custom form as 'Simple Contact Form' which conflicts with this plugin name Simple Contact Form. Try changing the name.
Edit1: You can also change the version if you don't want to change the name of the plugin. For eg: if you are prompted something like

There is a new version of Simple Contact Form. View version 14.13 details or update now.

then just rename the version of your plugin to 14.13 or above