Update example

[FedericoCeratto]

No description provided.

[Araq]

Use "" instead of nil please.

[skilchen]

@FedericoCeratto you should update the example once more since nil is no longer a valid value for strings. Currently your example produces:

let db = open("mytest.db", nil, nil, nil)
Error: type mismatch: got <string, nil, nil, nil>
but expected one of: 
proc open(f: var File; filehandle: FileHandle; mode: FileMode = fmRead): bool
  first type mismatch at position: 1
  required type: var File
  but expression '"mytest.db"' is of type: string
proc open(f: var File; filename: string; mode: FileMode = fmRead; bufSize: int = -1): bool
  first type mismatch at position: 1
  required type: var File
  but expression '"mytest.db"' is of type: string
proc open(filename: string; mode: FileMode = fmRead; bufSize: int = -1): File
  first type mismatch at position: 2
  required type: FileMode
  but expression 'nil' is of type: nil
proc open(connection, user, password, database: string): DbConn
  first type mismatch at position: 2
  required type: string
  but expression 'nil' is of type: nil

expression: open("mytest.db", nil, nil, nil)