Fix the description of the database argument to connect() (#847)

The docs incorrectly claim that the `database` argument defaults to the OS username, whereas in fact, the computed value of the `user` argument is used. Fixes: #823.

Author: elprans

asyncpg/connection.py

@@ -1841,7 +1841,7 @@ async def connect(dsn=None, *,
 
  If not specified, the value parsed from the *dsn* argument is used,
  or the value of the ``PGDATABASE`` environment variable, or the
- operating system name of the user running the application.
+ computed value of the *user* argument.
 
  :param password:
  Password to be used for authentication, if the server requires