Ch4 - description of isDirectory

[learningps]

In this section:
https://github.com/paf31/purescript-book/blob/master/text/chapter4.md#a-virtual-filesystem

The description of isDirectory says:

• The isDirectory function tests whether a function is a file or a directory.

I think it should say:

• The isDirectory function tests whether a Path is a file or a directory.