docs: tidy-up scheme references

After this patch `://` schemes are lowercase and enclosed in backticks.

Also:
- docs/libcurl/libcurl-multi.md: drop a stray C code fence.
- docs/libcurl/libcurl-tutorial.md: replace single/double quotes with
  Markdown markup where applicable.

Ref: #21646

Closes #21674

docs/examples/ftpsget.c

@@ -69,7 +69,7 @@ int main(void)
   if(curl) {
     /*
      * You better replace the URL with one that works! Note that we use an
-     * FTP:// URL with standard explicit FTPS. You can also do FTPS:// URLs if
+     * ftp:// URL with standard explicit FTPS. You can also do ftps:// URLs if
      * you want to do the rarer kind of transfers: implicit.
      */
     curl_easy_setopt(curl, CURLOPT_URL,