docs/devel/qapi-code-gen: Fix typo in grammar

An ALTERNATIVE's value can only be a type name.  Arrays are not
supported, yet.  The text gets it right: "The form STRING is shorthand
for { 'type': STRING }."  The grammar doesn't.  Fix it.

Fixes: b6c37eb
Reported-by: John Snow <jsnow@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200309142638.19988-1-armbru@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>

docs/devel/qapi-code-gen.txt

@@ -399,7 +399,7 @@ Syntax:
                   'data': ALTERNATIVES,
                   '*if': COND }
     ALTERNATIVES = { ALTERNATIVE, ... }
-    ALTERNATIVE = STRING : TYPE-REF
+    ALTERNATIVE = STRING : STRING
                 | STRING : { 'type': STRING, '*if': COND }
 
 Member 'alternate' names the alternate type.