docs: fix MANUAL formatting problems

1) Missing closing code backticks (```) in s3.md causes formatting problems
2) Pandoc requires blank lines before ATX headings

docs/content/commands/rclone_serve_ftp.md

@@ -33,6 +33,7 @@ then using Authentication is advised - see the next section for info.
 By default this will serve files without needing a login.
 
 You can set a single username and password with the --user and --pass flags.
+
 ## VFS - Virtual File System
 
 This command uses the VFS layer. This adapts the cloud storage objects