message: docs(aio): Fix incorrect wording
diff: @@ -139,7 +139,7 @@ Now the extractor tool and compiler will generate a translation unit with _your
  <code-example path="i18n/src/locale/messages.es.xlf.html" region="custom-id" linenums="false">
  </code-example>
  -Here is the `i18n` attribute with a _definition_, followed by the custom `id`:
  +Here is the `i18n` attribute with a _description_, followed by the custom `id`:
  <code-example path='i18n/src/app/app.component.1.html' region='i18n-attribute-id' title='app/app.component.html' linenums="false">
  </code-example>
  