folly test: fix typo in RandomDataHolder::RandomDataHolder

folly test: fix typo in RandomDataHolder::RandomDataHolder

Summary:
Running the folly/io/test:compression_test with a UBSAN-enabled binary would fail with this:

  folly/io/test/CompressionTest.cpp:96:36: runtime error: shift exponent 16777216 is too large for 64-bit type 'size_t' (aka 'unsigned long')

That exposed the unwanted left-shift.
That made it so the actual "start" offsets were in the range [0..numThreads-1], rather than being
e.g., with numThreads == 8, [0, 16777216, 33554432, ...].  Oops.

Reviewed By: luciang

Differential Revision: D3277606

fbshipit-source-id: 4bc48f84b0065de070b62a810f08b96e36ea7136