Fix typos in detectors documentation

[montyly]

https://github.com/CarstenNZ/slither-wiki/blob/master/Detector-Documentation.md has a couple of fixes / typos in the detectors description.

We should merge them before the next release

Related #1189

[le-anne]

Hey there- I'm new to open source and would love to try my hand at contributing to this one. Mind if I try to fix the typos & such?

[frazie]

I'd like to assist in fixing the errors alongside @le-anne