slightly simplify spi.yml file (#188)

Author: NicoHinderling

.spi.yml

@@ -1,11 +1,12 @@
+# This is manifest file for the Swift Package Index for it to auto-generate and
+# host DocC documentation.
+#
+# For reference see https://swiftpackageindex.com/swiftpackageindex/spimanifest/documentation/spimanifest/commonusecases#Host-DocC-documentation-in-the-Swift-Package-Index
 version: 1
 builder:
   configs:
     - documentation_targets:
-        - SnapshotPreferences
-        - Snapshotting
-        - SnapshottingTests
-    - platform: ios
-      scheme: PreviewGallery
-    - platform: macos
-      scheme: PreviewGallery
+      - SnapshotPreferences
+      - Snapshotting
+      - SnapshottingTests
+      - PreviewGallery