Cargo package - Fix missing domain in cargo sparse url (#27370)

Hello, it seems that one my previous PR (adding the sparse index to the
cargo package content page), did not worked as expected: the
gitea-origin-url does not add the AppURL because of the `sparse+` prefix
in the url.
Currently the rendered page gives the following:
```toml
[registry]
default = "gitea"

[registries.gitea]
index = "sparse+/api/packages/ownername/cargo/" # Sparse index
# index = "https://git.example.com/ownername/_cargo-index.git" # Git

[net]
git-fetch-with-cli = true
```

templates/package/content/cargo.tmpl

@@ -8,7 +8,7 @@
 default = "gitea"
 
 [registries.gitea]
-index = "<gitea-origin-url data-url="sparse+{{AppSubUrl}}/api/packages/{{.PackageDescriptor.Owner.Name}}/cargo/"></gitea-origin-url>" # Sparse index
+index = "sparse+<gitea-origin-url data-url="{{AppSubUrl}}/api/packages/{{.PackageDescriptor.Owner.Name}}/cargo/"></gitea-origin-url>" # Sparse index
 # index = "<gitea-origin-url data-url="{{AppSubUrl}}/{{.PackageDescriptor.Owner.Name}}/_cargo-index.git"></gitea-origin-url>" # Git
 
 [net]