Update exp_name in toke.c, following b5b6210a976

Ooops.

$  ./miniperl -DT -e 'undef'
### 0:LEX_NORMAL/XSTATE "\n"
### <== UNIOP(ival=op_undef)

### 1:LEX_NORMAL/XNumber too long ";"  <------ Look here!
### <== ';'

### 1:LEX_NORMAL/XSTATE ""
### Tokener got EOF
### <== EOF

EXECUTING...