docs: kdoc_parser: use '@' for Excess enum value

kdoc is looking for "@value" here, so use that kind of string in the
warning message. The "%value" can be confusing.

This changes:
Warning: drivers/net/wireless/mediatek/mt76/testmode.h:92 Excess enum value '%MT76_TM_ATTR_TX_PENDING' description in 'mt76_testmode_attr'

to this:
Warning: drivers/net/wireless/mediatek/mt76/testmode.h:92 Excess enum value '@MT76_TM_ATTR_TX_PENDING' description in 'mt76_testmode_attr'

Signed-off-by: Randy Dunlap <[e-mail]>
Reviewed-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Message-ID: <[e-mail]>

tools/lib/python/kdoc/kdoc_parser.py

···
         for k in self.entry.parameterdescs:
             if k not in member_set:
                 self.emit_msg(ln,
-                              f"Excess enum value '%{k}' description in '{declaration_name}'")
+                              f"Excess enum value '@{k}' description in '{declaration_name}'")
 
         self.output_declaration('enum', declaration_name,
                                 purpose=self.entry.declaration_purpose)