Update release notes with correct release date

sigmavirus24 · sigmavirus24 · commit ea3a2812876c · 2021-10-10T09:19:37.000-05:00
And this is the last bit of work for 4.0
diff --git a/HISTORY.rst b/HISTORY.rst
@@ -1,7 +1,7 @@
 Changelog - uritemplate
 =======================
 
-4.0.0 - 2021-10-xx
+4.0.0 - 2021-10-10
 ------------------
 
 - Drop support for Python 2.7, 3.4, and 3.5
