docs: fix typo in theme guidelines

[ReenigneArcher]

Description

Fix typo in docs and add emphasis to NOT.

Screenshot

Issues Fixed or Closed

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Dependency update (updates to dependencies)
• [x] Documentation update (changes to documentation)
• [ ] Repository update (changes to repository files, e.g. .github/...)

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch must be updated before it can be merged. You must also Allow edits from maintainers.

• [ ] I want maintainers to keep my branch updated

[codecov[bot]]

Codecov Report

Merging #2123 (8b1cf4c) into master (ceab515) will not change coverage. Report is 17 commits behind head on master. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2123 +/- ## ======================================= Coverage 85.98% 85.98% ======================================= Files 1 1 Lines 314 314 ======================================= Hits 270 270 Misses 44 44 ``` | [Files](https://app.codecov.io/gh/LizardByte/ThemerrDB/pull/2123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LizardByte) | Coverage Δ | | |---|---|---| | [src/updater.py](https://app.codecov.io/gh/LizardByte/ThemerrDB/pull/2123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LizardByte#diff-c3JjL3VwZGF0ZXIucHk=) | `85.98% <100.00%> (ø)` | |