DOC: additional join examples in "10 Minutes to pandas"
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Tutorial at [pandas.pydata.org/pandas-docs/stable/10min.html](http://pandas.pydata.org/pandas-docs/stable/10min.html) has the following code, where the obvious mistake is having only 1 key "foo" instead of 2 keys: "foo" and "bar" throughout the whole example 

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# Join
## SQL style merges. See the Database style joining

```
In [77]: left = pd.DataFrame({'key': ['foo', 'foo'], 'lval': [1, 2]})

In [78]: right = pd.DataFrame({'key': ['foo', 'foo'], 'rval': [4, 5]})

In [79]: left
Out[79]: 
   key  lval
0  foo     1
1  foo     2

In [80]: right
Out[80]: 
   key  rval
0  foo     4
1  foo     5

In [81]: pd.merge(left, right, on='key')
Out[81]: 
   key  lval  rval
0  foo     1     4
1  foo     1     5
2  foo     2     4
3  foo     2     5
```

Here is how it **should** be (replaced second 'foo' with 'bar':

![screenshot from 2016-04-29 14-49-45](https://cloud.githubusercontent.com/assets/5917788/14915186/ab7096ac-0e19-11e6-92f4-093f879824e4.png)
