Correct adjustment type application order in guide

[BenAkroyd]

Description
On the adjustments page of the guide, It states,

By default, tax adjustments are always applied before promotional adjustments.

The opposite is true. See https://github.com/solidusio/solidus/blob/master/core/app/models/spree/order_updater.rb#L111

Checklist:

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have updated Guides and README accordingly to this change (if needed)
• I have added tests to cover this change (if needed)

[kennyadsl]

@BenAkroyd thanks for the contribution! May I ask how did you verify that? The rest of the sentence makes me think that what is currently written is intentionally done to comply with some tax regulation. Can you help understand the context a little bit more, please?

[BenAkroyd]

@kennyadsl Sure! I've been working on tax integration for several months now with my company, and have had extensive conversations with our lawyer about how things should be calculated, and have seperately confirmed this with her.

[BenAkroyd]

The taxation page of the guide correctly lists the order of application https://github.com/solidusio/solidus/blob/master/guides/source/developers/taxation/overview.html.md#order-taxation

[kennyadsl]

@BenAkroyd oh well, it just makes sense. Thanks!

[BenAkroyd]

Also, In err, I included the corrected quote in the pr description, not the quote as it exists on the guide page currently. I have updated it.

[jacobherrington]

Thanks for the PR @BenAkroyd! Good catch 🥇

[BenAkroyd]

👍 thanks for the ultra-fast feedback!