Fix error message for missing key

[bewuethr]

This fixes the grammar in the error message for a missing key: "key %q not exists" to "key %q does not exist"

Link to the issue: n/a

Checklist

• I agree to follow the Code of Conduct by submitting this pull request.
• I have read and acknowledge the Contributing guide.
• I have added test cases to cover the new code.

I didn't add new test cases, but the behaviour is unchanged and tested here already:

ini/section_test.go

Lines 126 to 129 in b2f570e

	t.Run("key not exists", func(t *testing.T) {
	_, err := f.Section("").GetKey("404")
	require.Error(t, err)
	})

This test is independent of the error message itself.