command-line-interface: Fix 'MUST not' -> 'MUST NOT'

Fix text from e9a6625 (Clarity for commands vs global options,
2015-12-07) to use the MUST NOT phrasing defined by RFC 2119.  This
probably has the same meaning as "MUST not", but since RFC 2119
bothered to define the NOT forms we might as well use them.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

command-line-interface.md

@@ -11,7 +11,7 @@ $ funC [global-options] <COMMAND> [command-specific-options] <command-specific-a
 
 None are required, but the runtime MAY support options that start with at least one hyphen.
 Global options MAY take positional arguments (e.g. `--log-level debug`).
-Command names MUST not start with hyphens.
+Command names MUST NOT start with hyphens.
 The option parsing MUST be such that `funC <COMMAND>` is unambiguously an invocation of `<COMMAND>` (even for commands not specified in this document).
 If the runtime is invoked with an unrecognized command, it MUST exit with a nonzero exit code and MAY log a warning to stderr.