Fix typo in "Top Secret" exercise instructions

[jrgtt]

Hello, submitting a fix to a small typo I've spotted on the exercise instructions.

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[angelikatyborska]

Thanks, but this is not a typo. The instructions state:

If the operation in the AST node is defining a function, don't return the whole function name. Instead, check the function's arity. Then, return only first n character from the name, where n is the arity.

It's "ca" because the function cat(a, b) in the example takes two arguments.

[jrgtt]

Sorry, indeed!