Edit URL leads to 404 if you've specified a custom src directory #1543
Description
It looks like the code that generates the URL that users navigate to when clicking on the "Suggest an edit" button (added in #1506) assumes that your documentation is stored in a directory called src.
mdBook/src/renderer/html_handlebars/hbs_renderer.rs
Lines 40 to 50 in 7e7e779
As I've customised the directory where my documentation files are stored by using the src config option (to be docs/ instead), I end up getting a 404 when I click the "Suggest an edit" menu item button.
When I click the button, I'm lead to https://github.com/anoadragon453/examplerepo/edit/develop/src/README.md instead of https://github.com/anoadragon453/examplerepo/edit/develop/docs/README.md.
I believe the above code just needs to read from the value of the src config option, and default to "src" if it's not set.