fix typo, closes netbootxyz#1

fix typo, closes netbootxyz#1

Author: chris18890

README.md

@@ -8,7 +8,7 @@ It works by using your github user name that you input and chains to this url:
     https://raw.githubusercontent.com/${github_user}/netboot.xyz-custom/master/custom.ipxe
 
 Once forked, you can edit the menu as much as you want.  You can compile the iPXE image to
-set the github_user name early on so that you're github user name is set ahead of time and
+set the github_user name early on so that your github user name is set ahead of time and
 will automatically display your custom submenu on boot.  You can also set your github_user
 from the Utility menu which will cause a custom menu to appear in the main menu.