Bugfix in std.conv.parseEscape

This is a really easy and minor 1 liner fix:
std.conv.parseEscape basically forgot to check for backslash in the
list of possibly escaped characters. This made it imposible to parse
strings that container an actual backslash:

```D
import std.conv;

void main()
{
	string s = `["C:\\FolderA", "D:\\FolderB"]`;
	parse!(string[])(s); //Can't parse string: Unknown escape character \
}
```

This fixes that.