Update csvjoin.rst

removed  [-j JOIN] option from usage, not sure it is valid - this is my usage output:

```
$ pip show csvkit
---
Name: csvkit
Version: 0.9.0
Location: /usr/local/lib/python2.7/site-packages
Requires: xlrd, sqlalchemy, openpyxl, six, python-dateutil, dbf
$ csvjoin
usage: csvjoin [-h] [-d DELIMITER] [-t] [-q QUOTECHAR] [-u {0,1,2,3}] [-b]
               [-p ESCAPECHAR] [-z MAXFIELDSIZE] [-e ENCODING] [-S] [-v] [-l]
               [--zero] [-c COLUMNS] [--outer] [--left] [--right]
               [FILE [FILE ...]]
csvjoin: error: You must specify at least two files to join.
```