Fix minor typo in client_quickstart.rst (#3300)
Fix minor typo in client_quickstart.rst (#3300)

There has been a typo in URL-encoding example: a `/` was missing.
arthurdarcet/aiohttp,KeepSafe/aiohttp,KeepSafe/aiohttp,arthurdarcet/aiohttp,arthurdarcet/aiohttp,KeepSafe/aiohttp