Adding missing colon

Simple typo fix.
The absence of this colon may be confusing and result in misinterpretation of the result.

In normal libfuzzer mode, that colon is present.

You can compare with:
https://github.com/llvm/llvm-project/blob/aa0e9046c16bf27a8affbd903e2e3cad924a5217/compiler-rt/lib/fuzzer/FuzzerLoop.cpp#L356

Reviewed By: #sanitizers, vitalybuka

Differential Revision: https://reviews.llvm.org/D142171