Chronos: installation user guide typo (#4915)

* avoid installation of python 3.7.0 and typo

python 3.7.0 and 3.7.1 is problematic with Ray (happens while orca context being init.)

* go back to py 3.7

if base conda env is not py 3.7.0 or 3.7.1, then no pb.

docs/readthedocs/source/doc/Chronos/Overview/chronos.md

@@ -36,7 +36,7 @@ pip install bigdl-nano[tensorflow]
 pip install pmdarima==1.8.2
 pip install prophet==1.0.1
 pip install neural-compressor==1.8.1
-pip install pyarrow=6.0.1
+pip install pyarrow==6.0.1
 ```
 ```eval_rst
 .. note::