Spelling error corrected. (#8979)

<!--
  Q. Which branch should I use for my pull request?
  A. Use `master` branch (probably).

  Q. Which branch if my change is a bug fix for Gatsby v1?
  A. In this case, you should use the `v1` branch

  Q. Which branch if I'm still not sure?
  A. Use `master` branch. Ask in the PR if you're not sure and a Gatsby maintainer will be happy to help :)

  Note: We will only accept bug fixes for Gatsby v1. New features should be added to Gatsby v2.

  Learn more about contributing: https://www.gatsbyjs.org/docs/how-to-contribute/
-->

CHANGELOG.md

@@ -267,7 +267,7 @@
 - Add missing npmignore files #1298 @kyleamathews
 - Move creating 404.html page into plugin so can enforce pages only created by
   plugins #1300 @kyleamathews
-- Fix occassionally out-of-order query watching which would throw errors #1301
+- Fix occasionally out-of-order query watching which would throw errors #1301
   @kyleamathews
 - Waiting for query extraction wasn't actually waiting #1303 @kyleamathews
 - Improved plugin error without exit #1309 @0x80