Fix watch file.

debian/changelog

@@ -1,6 +1,7 @@
 dulwich (0.20.15-2) UNRELEASED; urgency=medium
 
   * Update watch file format version to 4.
+  * Fix watch file.
 
  -- Jelmer Vernooij <jelmer@debian.org>  Wed, 03 Mar 2021 17:14:44 -0000
 

debian/watch

@@ -1,2 +1,2 @@
 version=4
-opts=pgpsigurlmangle=s/$/.asc/ https://pypi.debian.net/dulwich dulwich-(.*).tar.gz
+opts=pgpsigurlmangle=s/$/.asc/ https://pypi.debian.net/dulwich/dulwich-(.*).tar.gz