dart pub compat: do not use gitignore rules starting with '/'.

Author: greenrobot-team

objectbox/example/flutter/objectbox_demo/android/.gitignore

@@ -1,9 +1,11 @@
 gradle-wrapper.jar
-/.gradle
-/captures/
-/gradlew
-/gradlew.bat
-/local.properties
+### These rules should only apply to current directory, but dart pub ignores rules that start with /
+.gradle
+captures/
+gradlew
+gradlew.bat
+local.properties
+###
 GeneratedPluginRegistrant.java
 
 # Remember to never publicly share your keystore.