fix: increment header z-index to show above everything else

[Rex-82]

fixes: #67

Description

On the pricing page the header goes behind some text elements when scrolling.
Increasing the z-index value from 10 to 20 for the containing element fixes the issue

Contribution Checklist:

• The pull request only addresses one issue or adds one feature.
• The pull request does not introduce any breaking changes
• I have added screenshots or gifs to help explain the change if applicable.
• I have read the contribution guidelines.
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