Updated documentation to include mandatory params


Examples were not working,

iex(23)> Postgrex.Connection.query(pid, "CREATE TABLE posts (id serial, title text)") 
** (UndefinedFunctionError) undefined function: Postgrex.Connection.query/2
    Postgrex.Connection.query(#PID<0.228.0>, "CREATE TABLE posts (id serial, title text)")

Looking at the code it seems like the params is no longer optional, so I updated the documentation (the other option is to continue to make the params optional)

Also note that the $1 + $2 query was throwing type exceptions in my version of postgres (9.3), so I changed it slightly.