contrib: restore the `hg fix` configuration in the examples

After decc3bd3f20d, running `black` will DTRT, but running `hg fix` did nothing
(unless the example config file was %included, in which case it truncated the
file instead of formatting it).  I'm not sure why that was happening, but let's
not leave a code shredder laying around.

Differential Revision: https://phab.mercurial-scm.org/D10311

contrib/examples/fix.hgrc

@@ -1,14 +1,14
 [fix]
 clang-format:command = clang-format --style file
 clang-format:pattern = set:(**.c or **.cc or **.h) and not "include:contrib/clang-format-ignorelist"
 
 rustfmt:command = rustfmt +nightly
 rustfmt:pattern = set:"**.rs" - "mercurial/thirdparty/**"
 
-black:command = black
+black:command = black --config=pyproject.toml -
 black:pattern = set:**.py - mercurial/thirdparty/**
 
 # Mercurial doesn't have any Go code, but if we did this is how we
 # would configure `hg fix` for Go:
 go:command = gofmt
 go:pattern = set:**.go