Specify screenshot files only, not whole folder

The whole screenshots folder shouldn't be ignore since there could be frameit configuration in it (Framefile.json, fonts, background.jpg, *.strings, etc...). It'd be better to just ignore the generated PNG screenshots.

Swift.gitignore

@@ -64,5 +64,5 @@ Carthage/Build
 
 fastlane/report.xml
 fastlane/Preview.html
-fastlane/screenshots
+fastlane/screenshots/*/*.png
 fastlane/test_output