Fix three small but clear errors in documentation:
benchmarking.md:"coins selection"→"coin selection"(wrong plural)tracing.md:"succeeded asbool"→"succeeded asbool"(missing space breaks backtick formatting)assumeutxo.md:"For most of the duration of dumptxoutset running"→"For most of the duration that dumptxoutset is running,"(awkward phrasing, missing comma)