fix: correct typo in "Auth External API" example (#3919)

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## Types of changes
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## Description
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Corrected `crendentials` to `credentials` in the ["Auth External API" example](https://github.com/nuxt/nuxt.js/tree/dev/examples/auth-jwt). Text-only change.

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examples/auth-jwt/pages/login.vue

@@ -9,7 +9,7 @@
         <input id="password" type="password" value="test">
       </label>
       <button @click="postLogin">login</button>
-      <p>The crendentials are not verified for the example purpose.</p>
+      <p>The credentials are not verified for the example purpose.</p>
     </div>
   </div>
 </template>