added the step of validating the adaptor signature

[ChrisSon15]

The atomic swap was missing the step of validating the adaptor signature. IMHO, This should be added as otherwise it would be misleading to readers who seek education on this subject. I saw the warning sign, but I think, if you describing in a high level, then at least the high level would have all required steps. Also I made it a bit clearer that there is a second adaptor signature for the swap. Please consider reviewing.

[ChrisSon15]

Hi @jonasnick , Sorry for combining 2 changes into one. To separate them I changed this PR to only add the adaptor signature verification step explicitly. As for the need of 2 adaptor signatures for s swap I may put it into another issue.