Fix Chart doc build (#18302)
Fix Chart doc build (#18302)

Error on main:

```
 WARNING: Title underline too short.
```
Acehaidrey/incubator-airflow,apache/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,bolkedebruin/airflow,lyft/incubator-airflow,apache/airflow,apache/airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,mistercrunch/airflow,apache/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,apache/incubator-airflow,cfei18/incubator-airflow,mistercrunch/airflow,lyft/incubator-airflow,mistercrunch/airflow,bolkedebruin/airflow,apache/incubator-airflow,bolkedebruin/airflow,bolkedebruin/airflow,nathanielvarona/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,apache/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,bolkedebruin/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow