README: fix "Pragmatism" link target (#2901)

Fixes #2897

README.md

@@ -81,7 +81,9 @@ section for details). If you're feeling confident, use `--fast`.
 
 _Black_ is a PEP 8 compliant opinionated formatter. _Black_ reformats entire files in
 place. Style configuration options are deliberately limited and rarely added. It doesn't
-take previous formatting into account (see [Pragmatism](#pragmatism) for exceptions).
+take previous formatting into account (see
+[Pragmatism](https://black.readthedocs.io/en/stable/the_black_code_style/current_style.html#pragmatism)
+for exceptions).
 
 Our documentation covers the current _Black_ code style, but planned changes to it are
 also documented. They're both worth taking a look: