Use "parameter", not "argument", in Lambda type

[showell]

Technically an argument is what you pass in, and the parameter defines the var in the function definition. I know people use these as synonyms, but it might make some code cleaner when you can distinguish parameter names (i.e. Lambda) from args (i.e. Call).

[Janiczek]

I believe it can be argued both ways. "type of the argument", "argument type" are what's meant in the current code. If we use "type argument", that's wrong for sure. But lambdas have arguments, not parameters.

I'll mull it over during the weekend! :) @showell Feel free to add specific places in the code where you believe the proposed change would add clarity.