update_prebuilts: fix incorrectly formatted extensions/README.md entry

packages/modules/common/tools/finalize_sdk.py adds the leading "-
<sdk-ext-version>" part itself, so no need for update_prebuilts.py to
pass in that part.

Test: ./prebuilts/sdk/update_prebuilts.py -f 35 -e 13 --bug 1234 <build-id>
Bug: 340822159
Change-Id: I85b0c169a55c8aa3f5106beb05858bb615a0b1e8
1 file changed