Fix order of arguments to GetRelativePath

These have been backwards since the feature was added (5f2e2f39b). Does the
Windows implementation of GetRelativePath not return weird results?

src/BizHawk.Client.EmuHawk/tools/MultiDiskBundler/MultiDiskBundler.cs

@@ -213,7 +213,7 @@ namespace BizHawk.Client.EmuHawk
 						new XElement("LoadAssets",
 							names.Select(n => new XElement(
 								"Asset",
-								new XAttribute("FileName", PathExtensions.GetRelativePath(basePath, n))
+								new XAttribute("FileName", PathExtensions.GetRelativePath(n, basePath))
 							))
 						)
 					);