[doc] add ellipsis atom to baron grammar file

grammar/baron_grammar

@@ -99,7 +99,7 @@ atom: ('(' [yield_expr|testlist_comp] ')' |
        '[' [listmaker] ']' |
        '{' [dictorsetmaker] '}' |
        '`' testlist1 '`' |
-       NAME | NUMBER | STRING+)
+       NAME | NUMBER | STRING+ | '...')
 testlist_comp: test ( comp_for | (',' test)* [','] )
 listmaker: test ( list_for | (',' test)* [','] )
 trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME