DOC: correct an example in v0.14.0.rst to fix "NameError"
while building whatsnew/v0.14.0.rst the error occurs:
```
----> 1 g[['B']].head(1)

NameError: name 'g' is not defined
```
corrected the example to fix the error.
@rhshadrach, could you please take a look at this PR? Thanks @natmokval  Thanks @rhshadrach for reviewing this PR.