Update documentation/manual/detailledTopics/configuration/ThreadPools.md

A small typo as mentioned in #721
Merge pull request #722 from ndeverge/patch-4

Update documentation/manual/detailledTopics/configuration/ThreadPools.md
Merge commit 'bcf94f574dd84d62ae8b46838262ca288d67c6d5' into 2.1.x