Minor README fixes (#17532)
Minor README fixes (#17532)

- Markdown was not formatted correctly in the in-line code-block
- Just adds single quotes around pip install as the square bracket errors out on some terminals
bolkedebruin/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,apache/airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,apache/incubator-airflow,lyft/incubator-airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,dhuang/incubator-airflow,mistercrunch/airflow,bolkedebruin/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,nathanielvarona/airflow,apache/airflow,apache/airflow,mistercrunch/airflow,nathanielvarona/airflow,apache/airflow,mistercrunch/airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,dhuang/incubator-airflow,cfei18/incubator-airflow,bolkedebruin/airflow,apache/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,apache/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,apache/airflow,mistercrunch/airflow,lyft/incubator-airflow,apache/incubator-airflow