Update SwiftPackageManager.md (#7297)

Workaround for finding Crashlytics run script when using SPM.

SwiftPackageManager.md

@@ -51,8 +51,8 @@ https://github.com/firebase/firebase-ios-sdk/issues/6472#issuecomment-694449182.
 
 <img src="docs/resources/SPMObjC.png">
 
-If you're using FirebaseCrashlytics, use
-`${BUILD_DIR}/SourcePackages/checkouts/firebase-ios-sdk/Crashlytics/run`
+If you're using FirebaseCrashlytics, you can use
+`${BUILD_DIR%Build/*}/SourcePackages/checkouts/firebase-ios-sdk/Crashlytics/run`
 as the run script that allows Xcode to upload your project's dSYM files.
 
 Another option is to use the