Fix typo in config_description hash key

[tobias-bischoff]

Typo “seriserialization_options” → “serialization_options”. I couldn't find any references to this misspelt key in the current codebase, neither any GitHub search result besides forks of jsonapi-resources 😄 So I presume it's safe to correct it.

The only usage of the affected config_description method I found ends up using the keys as part of a cache key, which won't be affected either.

closes #1273

All Submissions:

• I've checked to ensure there aren't other open Pull Requests for the same update/change.
• I've submitted a ticket for my issue if one did not already exist.
• My submission passes all tests. (Please run the full test suite locally to cut down on noise from travis failures.)
• I've used Github auto-closing keywords in the commit message or the description.
• I've added/updated tests for this change.

Bug fixes and Changes to Core Features:

• I've included an explanation of what the changes do and why I'd like you to include them.
• I've provided test(s) that fails without the change.

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions

[lgebhardt]

@tobias-grasse Thanks!