INSTALL.md: correct a filename with mixed underscore and dash

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>

INSTALL.md

@@ -73,7 +73,7 @@ filter-repo only consists of a few files that need to be installed:
     of filter-repo as a python library.
 
     You can create this symlink to (or copy of) git-filter-repo named
-    git_filter-repo.py and place it in your python site packages; `python
+    git_filter_repo.py and place it in your python site packages; `python
     -c "import site; print(site.getsitepackages())"` may help you find the
     appropriate location for your system.  Alternatively, you can place
     this file anywhere within $PYTHONPATH.