dispatcher: fixing some misspelled words in code/docs

First of all excuse me if in the previous pull request I didn't follow guidelines by devel-git-commit guide :)
As anticipated in the previous request I made some changes to dispatcher module for counting successful ping requests before moving a destination from inactive to active state.
While doing this changes I noticed some misspelled variables (in code and docs too) and I fixed them. So I've used similar variables (with the corrected words) also for the new feature I developed. 
In the case we leave the old variables introducing the new one we will have in the code:
"ds_probing_threshhold" vs "ds_inactive_threshold"
This behaviour could be confusing while editing the configuration file and also while reading the code.

As you requested I've separated the misspelled fixes in two commits one for the code and one for the docs.
To this pull request will follow the one with the changes developing the new feature.