Switch Black link to documentation.

README.md

@@ -382,9 +382,9 @@ the following deviations from it:
  * Within multiline constructs, use standard four-space indentation. Don't align
    indentation with an opening delimeter.
 
-borgmatic code uses the [Black](https://github.com/ambv/black) code formatter,
-so some additional code style requirements will be enforced as well. See the
-Black documentation for more information.
+borgmatic code uses the [Black](https://black.readthedocs.io/en/stable/) code
+formatter, so some additional code style requirements will be enforced as
+well. See the Black documentation for more information.
 
 
 ### Development
@@ -441,8 +441,9 @@ Note that while running borgmatic itself only requires Python 3+, running
 borgmatic's tests require Python 3.6+.
 
 If when running tests, you get an error from the
-[Black](https://github.com/ambv/black) code formatter about files that would
-be reformatted, you can ask Black to format them for you via the following:
+[Black](https://black.readthedocs.io/en/stable/) code formatter about files
+that would be reformatted, you can ask Black to format them for you via the
+following:
 
 ```bash
 tox -e black