fix(docs): fix variable name in README.md

[JonasLaux]

fixed naming of variable

Fixes #<issue_number_goes_here> (it's a good idea to open an issue first for discussion)

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

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[bcoe]

@JonasLaux thanks for the contribution 👍 could I bother you to sign the CLA?

[JonasLaux]

Hey,
yes i already did it, after the PR

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