DOC: fixup link for lookup example
#### Location of the documentation

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


#### Documentation problem

It says

> For an example see lookup() in the user guide.

but the hyperlink doesn't send to the user guide

#### Suggested fix for documentation

link to https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#looking-up-values-by-index-column-labels instead
