fix: fix inconsistent definition of assertError function in maps section

[dario-piotrowicz]

The assertError in the maps section is introduced here:

learn-go-with-tests/maps.md

Lines 222 to 228 in 7c5b39c

	func assertError(t testing.TB, got, want error) {
	t.Helper()
	if got != want {
	t.Errorf("got error %q want %q", got, want)
	}
	}

as you can see with the failure error being got error "X" want "Y"

This does not match the v1 go code:

learn-go-with-tests/maps/v1/dictionary_test.go

Line 18 in 7c5b39c

t.Errorf("got %q want %q", got, want)

Further down the section the assertError helper is refactored:

learn-go-with-tests/maps.md

Lines 367 to 375 in 7c5b39c

	func assertError(t testing.TB, got, want error) {
	t.Helper()
	if got != want {
	t.Errorf("got %q want %q", got, want)
	}
	}
	```
	For this test, we modified `Add` to return an error, which we are validating against a new error variable, `ErrWordExists`. We also modified the previous test to check for a `nil` error, as well as the `assertError` function.

to update the failure error to got "X" want "Y" (without error)

This does not really make much sense and is also not reflected anywhere in the section's go code (see: v2, v3, ..., v7)

So this PR is amending the erroneous v1 go code and removing the unnecessary/incorrect refactoring