Update size of the checkout in the contribution guide.
Update size of the checkout in the contribution guide.

Test: None
Change-Id: Iafc951d6a8f955b60f1dbec139e850c19be1af50

AndroidX/androidx,androidx/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,androidx/androidx,androidx/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx,aosp-mirror/platform_frameworks_support,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,aosp-mirror/platform_frameworks_support,AndroidX/androidx,AndroidX/androidx,androidx/androidx,aosp-mirror/platform_frameworks_support,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx