Fix a silly bug. Sigh.

Change-Id: Ia67342d56907053228895ed6cd65c98949bd7c43
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63352
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>

python/setup.py

diff --git a/python/setup.py b/python/setup.py
index e695cb0..e128cf9 100644
--- a/python/setup.py
+++ b/python/setup.py
@@ -117,7 +117,7 @@
   ):
     with open(filename, 'r') as file:
       contents = file.read()
-    filename = re.sub(r'^re2(?=\.py)', '__init__', contents)
+    filename = re.sub(r'^re2(?=\.py)', '__init__', filename)
     contents = re.sub(r'^(?=import _)', 'from . ', contents, flags=re.MULTILINE)
     with open(f'{PACKAGE}/{filename}', 'x') as file:
       file.write(contents)