stbt.android.AdbDevice.get_frame: Fix warning message
stbt.android.AdbDevice.get_frame: Fix warning message

I changed the number of attempts in 1471b16 ("AdbDevice.get_frame: Only
retry 3 times") but I didn't update this message.

LewisHaley/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester