[doc] Fix incorrect argument syntax for --timeout-factor

Summary: Missing the equals fails to run.

Test Plan:

./test/functional/test_runner.py --timeout-factor=0.3 abc_p2p_avalanche_voting

Reviewers: #bitcoin_abc, majcosta, Fabien

Reviewed By: #bitcoin_abc, majcosta, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12892