Fix a typo related to abidiffs

This commit fixes a typo related to abidiffs.  The generated abidiffs
should be copied to `$$DIST_DIR/abidiffs` instead of
`$$DIST_DIR/abidiff`.

(See also. https://android-review.googlesource.com/796458/)

Bug: 123491909
Test: lunch aosp_x86-userdebug && make dist
Change-Id: I89c13580344a27cac5ea0d00497f5bba3227cf61