Move note about default.nix to discussion of imports

[tejing1]

This:

nix-book/src/ch05-01-language-basics.md

Lines 19 to 20 in f6448f4

	*NOTE*: If a path to a directory is passed, but a file was expected; then nix will look
	for a `default.nix` located in that directory, and will target that.

Is really a property of import, not path-types in general. It should go where imports are being discussed.

[jonringer]

Yea, I realized that when addressing #12 with 9e1afce

Thanks for the correction :)