Edit URL leads to 404 if you've specified a custom src directory

[anoadragon453]

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

	if let Some(ref edit_url_template) = ctx.html_config.edit_url_template {
	let full_path = "src/".to_owned()
	+ ch.source_path
	.clone()
	.unwrap_or_default()
	.to_str()
	.unwrap_or_default();
	let edit_url = edit_url_template.replace("{path}", &full_path);
	ctx.data
	.insert("git_repository_edit_url".to_owned(), json!(edit_url));
	}

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.