chore: fix typo in middleware readme

[Vinesse]

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Fix a spelling error in the middleware readme file.

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly. (PR: #)
• I have added tests to cover my changes (if not applicable, please state why)
  (not applicable as it's simply a spelling update)
• All new and existing tests are passing.