Typo fix in prebuilt.py.
Typo fix in prebuilt.py.

BUG=chromium-os:10175
TBR=build breaker
TEST=Run prebuilt.py with --previous-binhost-url set

Change-Id: I2fa2edf88e30f0cc53e3807291f5bf7ce58a79e5

Review URL: http://codereview.chromium.org/6463029

zhang0137/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite,coreos/chromite,chadversary/chromiumos.chromite,chadversary/chromiumos.chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,coreos/chromite