fix error on Mac when a custom map references a vanilla tilesheet that only exists under Content/Maps

docs/release-notes.md

@@ -3,6 +3,7 @@
 These changes have not been released yet.
 
 * For players:
+  * Fixed error when a custom map references certain vanilla tilesheets on MacOS.
   * Fixed compatibility with Arch Linux.
 
 ## 2.11.1

src/SMAPI/Framework/ModHelpers/ContentHelper.cs

@@ -316,7 +316,7 @@ namespace StardewModdingAPI.Framework.ModHelpers
 
             // check relative to content folder
             {
-                foreach (string candidateKey in new[] { imageSource, $@"Maps\{imageSource}" })
+                foreach (string candidateKey in new[] { imageSource, Path.Combine("Maps", imageSource) })
                 {
                     string contentKey = candidateKey.EndsWith(".png")
                         ? candidateKey.Substring(0, candidateKey.Length - 4)