Fix spelling in docs sidebar

[Strepto]

Description

Fixes the spelling of RestSharp on the intro page at https://restsharp.dev/docs/intro

Purpose

This pull request is a:

• Bugfix (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)
• Other

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)