fix(init): don't gitignore triple-dash

The `---` is only for the `README.md` file, not `gitignore`.

I've also switched the "(ab)used link label (for use with reference
style links)"-style comment (`[//] #`) for an HTML comment (`<!-- ...
-->`). This means it will be rendered in the HTML, which might be
preferable, and is at a minimum benign. I hadn't been familiar with the
`[//]: #`-style comment until I saw it here (and searched to find a
description of it, along with the "(ab)use" line above, [here]), and I
don't think it's worth the potential confusion for users of `soroban
contract init`.

  [here]: https://stackoverflow.com/a/20885980/249801

cmd/soroban-cli/src/commands/contract/init.rs

@@ -422,14 +422,14 @@ fn append_contents(from: &Path, to: &Path) -> Result<(), Error> {
 
 fn get_merged_file_delimiter(file_path: &Path) -> String {
     let comment = if file_path.to_string_lossy().contains("README.md") {
-        "[//]: # \"The following is the Frontend Template's README.md\"".to_string()
+        "---\n<!-- The following is the Frontend Template's README.md -->".to_string()
     } else if file_path.to_string_lossy().contains("gitignore") {
         "# The following is from the Frontend Template's .gitignore".to_string()
     } else {
         String::new()
     };
 
-    format!("\n\n---\n\n{comment}\n\n").to_string()
+    format!("\n\n{comment}\n\n").to_string()
 }
 
 #[cfg(test)]