Correct usage of invalid file URL in test.

Author: freakboy3742

tests/functional/test_bad_url.py

@@ -8,7 +8,7 @@ def test_filenotfound_error_message(script: Any) -> None:
     # Test the error message returned when using a bad 'file:' URL.
     # make pip to fail and get an error message
     # by running "pip install -r file:nonexistent_file"
-    proc = script.pip("install", "-r", "file:unexistent_file", expect_error=True)
+    proc = script.pip("install", "-r", "file:///unexistent_file", expect_error=True)
     assert proc.returncode == 1
     expect = (
         "ERROR: 404 Client Error: FileNotFoundError for url: file:///unexistent_file"