docs: Fixed instructions on checking VM version.

Per IRogers@ , you check the VM version by calling
System.getProperty("java.vm.version") -- not, as previously
documented, "dalvik.vm.version" (Ian says there's no such property as
dalvik.vm.version.) BDC@ and/or AnwarG@, can you confirm?

Revised doc is staged to:

http://asolovay.mtv:9001/guide/practices/verifying-apps-art.html#GC_Migration

bug: 15836664
Change-Id: Ie5e2e058fa9ce033afd18a0f4079faa81bbf317e