formatting: remove the data-ogre from the config example

Without he final `-`, black ignore the stdin and just looks at file provided as
command line argument.

Since `hg fix` feeds the file content through stdin and does not pass file
argument, this meant black happily exited successful (all files passed as
argument were formatted) without any output. Fix picked this "no output" as the
new file content, deleting all previous content.

I appreciate the fact this effectively removed all buggy code in any files
processing that way, but this also ate all my data.

The example config is now fixed in that regards.

contrib/examples/fix.hgrc

@@ -1,15 +1,15 b''
 [fix]
 clang-format:command = clang-format --style file -i
 clang-format:pattern = (**.c or **.cc or **.h) and not "listfile:contrib/clang-format-ignorelist"
 
 rustfmt:command = rustfmt {rootpath}
 rustfmt:pattern = set:**.rs
 
 # We use black, but currently with
 # https://github.com/psf/black/pull/826 applied. For now
 # contrib/grey.py is our fork of black. You need to pip install
 # git+https://github.com/python/black/@d9e71a75ccfefa3d9156a64c03313a0d4ad981e5
 # to have the dependencies for grey.
 #
-# black:command = python3.7 contrib/grey.py --skip-string-normalization
+# black:command = python3.7 contrib/grey.py --skip-string-normalization -
 # black:pattern = set:**.py - hgext/fsmonitor/pywatchman/** - mercurial/thirdparty/** - "contrib/python-zstandard/**"