docs: fix typo in calculator function description

[dsmedia]

Corrected a typo in the documentation. Changed "indepent" to "independent" in the description of defining the calculator function.

Before: "The first step is to define the actual calculator function and make sure it works, indepent of Claude. We'll write a VERY simple function that expects three arguments:\n"

After: "The first step is to define the actual calculator function and make sure it works, independent of Claude. We'll write a VERY simple function that expects three arguments:\n"