DOC: incorrect example result for pd.merge_ordered

[ZhuBaohe]

Exsample result for 'merge_ordered' function in documentation is wrong.
https://pandas-docs.github.io/pandas-docs-travis/generated/pandas.merge_ordered.html?highlight=merge_ordered#pandas.merge_ordered

The result should be:

key lvalue group rvalue
0 a 1 a NaN
1 b 1 a 1.0
2 c 2 a 2.0
3 d 2 a 3.0
4 e 3 a 3.0
5 a 1 b NaN
6 b 1 b 1.0
7 c 2 b 2.0
8 d 2 b 3.0
9 e 3 b 3.0

[jreback]

This is probably some old copy-paste text.

Would you do a PR to update and add a test to validate this

In [8]:         left = DataFrame({'key': ['a', 'c', 'e', 'a', 'c', 'e'],
   ...:                                'lvalue': [1, 2, 3, 1, 2, 3], 'group': list('aaabbb')})
   ...: 
   ...:         right = DataFrame({'key': ['b', 'c', 'd'],
   ...:                                 'rvalue': [1, 2, 3]}
   ...:                        
   ...:                        )
   ...: 

In [9]: 

In [9]: pd.merge_ordered(left, right, fill_method='ffill', left_by='group')
Out[9]: 
  group key  lvalue  rvalue
0     a   a       1     NaN
1     a   b       1     1.0
2     a   c       2     2.0
3     a   d       2     3.0
4     a   e       3     3.0
5     b   a       1     NaN
6     b   b       1     1.0
7     b   c       2     2.0
8     b   d       2     3.0
9     b   e       3     3.0