DOC: Typo fix in ordered_merge warning
## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/14271?src=pr) is 85.25% (diff: 100%)

> Merging [#14271](https://codecov.io/gh/pydata/pandas/pull/14271?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #14271   diff @@
==========================================
  Files           140        140          
  Lines         50568      50579    +11   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43112      43123    +11   
  Misses         7456       7456          
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [a7469cf...64e97d6](https://codecov.io/gh/pydata/pandas/compare/a7469cf98275a183ad2e4bfafa9706a1ef8d035e...64e97d6921a73d27ec0a20e289216b61524a85cf?src=pr)
 @jerenrich Thanks!
