Fix capitalisation of "macNumbersTest.xlsx" in the code, in order to make tests pass.

macNumbers_test.go

@@ -7,7 +7,7 @@ import (
 // Test that we can successfully read an XLSX file generated by
 // Numbers for Mac.
 func TestMacNumbers(t *testing.T) {
-	xlsxFile, error := OpenFile("MacNumbersTest.xlsx")
+	xlsxFile, error := OpenFile("macNumbersTest.xlsx")
 	if error != nil {
 		t.Error(error.Error())
 		return