protocol-v2.txt: align delim-pkt spec with usage

The current protocol EBNF allows command-request to end with the
capability list, if no command specific arguments follow, but the
protocol requires that after the capability list, there must be a
delim-pkt regardless of the number of command specific arguments.  Fixed
the EBNF to match. Both JGit and libgit2's implementation has the
delim-pkt as mandatory. JGit's code is not publicly linkable, but
libgit2 is linked below[1]. As for currently implemented commands on v2
(ls-ref and fetch), the delim packet is already being passed through

[1]: https://github.com/libgit2/libgit2/blob/main/src/transports/git.c

Reported-by: Ivan Frade <ifrade@google.com>
Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/protocol-v2.txt

@@ -116,11 +116,11 @@ command can be requested at a time.
     empty-request = flush-pkt
     command-request = command
 		      capability-list
-		      [command-args]
+		      delim-pkt
+		      command-args
 		      flush-pkt
     command = PKT-LINE("command=" key LF)
-    command-args = delim-pkt
-		   *command-specific-arg
+    command-args = *command-specific-arg
 
     command-specific-args are packet line framed arguments defined by
     each individual command.