[AIRFLOW-XXX] Fix doc of command line interface

[XD-DENG]

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There are two issues found in documentation of command line interface

Issue 1

The default value of --subdir is generated dynamically, so Kaxil's Dag Folder during compiling is used in the final documentation (this appeared 14 times in https://airflow.apache.org/cli.html). I believe this should be "hardcoded" into [AIRFLOW_HOME]/dags.

Issue 2
A minor typo (a space is missing).

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[ashb]

Unfortunatley changing this default actually changes the value that will be in args.subdir - so this change would break a lot of things.

It might be possible to change the $AIRFLOW_HOME when rendering the docs, or to change the display of this param?

@kaxil How are the docs built when render then? Would it maybe be worth setting something like export AIRFLOW_HOME=\$AIRFLOW_HOME so that these defaults make sense? (That might well break other things)

[XD-DENG]

You're right. Didn't realize this.

I would leave this to @kaxil to decide how he would like to fix this (please feel free to close this PR should you're going to fix this separately).

But we do need to fix this. Default: /Users/kaxil/airflow/dags in the documentation is very confusing and misleading to doc readers.

[kaxil]

Good catch @XD-DENG . Wonder how long this issue went un-noticed. We have this issue in 1.9.0, 1.8.2 docs as well:

1.9.0: https://airflow.readthedocs.io/en/1.9.0/cli.html#Named%20Arguments_repeat4
1.8.2: https://airflow-fork-k1.readthedocs.io/en/v1-8-stable/cli.html#Named%20Arguments_repeat1

I fear we had this Bug since the start.

@ashb you are right, changing this will cause issues. I will create a new PR to take care of this.

[XD-DENG]

Thanks @kaxil for the further checking.

I'll close this PR then. Regarding issue 2 (a missing space), may you fix that in your new PR as well? Thanks!