fix(lint): update darker version and README

Author: dshemetov

README.md

@@ -98,7 +98,7 @@ through black. You can do that with the following command (using the same
 virtual environment as above):
 
 ```sh
-black <file>
+env/bin/black <file>
 ```
 
 # COVIDcast

requirements.lint.txt

@@ -1,7 +1,6 @@
 # Keep versions in sync with cmu-delphi/covidcast-indicators
-darker[flynt]~=1.7.2
+darker[isort]~=2.1.1
 invoke>=1.4.1
-isort==5.12.0
 lint_diffs==0.1.22
 pylint==2.8.3
 requests