DOC: downcast argument for pd.to_numeric should be integer, not int
#### Location of the documentation

https://pandas.pydata.org/docs/dev/reference/api/pandas.to_numeric.html?highlight=to_numeric#pandas.to_numeric

#### Documentation problem

The documentation declares that the `downcast` argument of `pd.to_numeric` can use the value `int`. However, only `integer` is working in the real code:

```
In [8]: pd.to_numeric(["42"], downcast="int", errors="coerce")
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-8-3b178e77dcd5> in <module>
----> 1 pd.to_numeric(["42"], downcast="int", errors="coerce")

/usr/lib/python3.8/site-packages/pandas/core/tools/numeric.py in to_numeric(arg, errors, downcast)
    110     """
    111     if downcast not in (None, "integer", "signed", "unsigned", "float"):
--> 112         raise ValueError("invalid downcasting method provided")
    113 
    114     if errors not in ("ignore", "raise", "coerce"):

ValueError: invalid downcasting method provided

In [9]: pd.to_numeric(["42"], downcast="integer", errors="coerce")
Out[9]: array([42], dtype=int8)
```

#### Suggested fix for documentation

Replace `int` with `integer`