introduction.md: fix wrong term describing assignment operator

[amavrin]

"assignment operator assigns a variable with a literal" seems to be incorrect. Better to say "assignment operator assigns a variable with a value"

[github-actions]

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[amavrin]

https://forum.exercism.org/t/freelancer-rates-introduction-md-fix-wrong-term-describing-assignment-operator/56464

the assignment operator assigns a value to a variable is much easier to parse