Editorial: fix typo around "mutual exclusive" (#1754)

This is for addressing #1716.
I also found that "not" should also be an exclusive condition.
Let me update.

Author: yoshisatoyanagisawa

docs/index.bs

@@ -3455,7 +3455,7 @@ spec: storage; urlPrefix: https://storage.spec.whatwg.org/
           1. Return true.
       1. Else:
 
-          Note: The [=Verify Router Condition=] algorithm guarantees that {{RouterCondition/or}} and other conditions are mutual exclusive.
+          Note: The [=Verify Router Condition=] algorithm guarantees that {{RouterCondition/or}}, {{RouterCondition/not}}, and other conditions are mutually exclusive.
 
           1. If |condition|["{{RouterCondition/urlPattern}}"] [=map/exists=], then:
               1. Let |rawPattern| be |condition|["{{RouterCondition/urlPattern}}"].