Fixed whatsnew PR link.

Author: akuhnregnier

docs/src/whatsnew/latest.rst

@@ -105,7 +105,7 @@ This document explains the changes made to Iris for this release
    environment info. (:pull:`3990`)
 
 #. `@akuhnregnier`_ replaced `deprecated numpy 1.20 aliases for builtin types`_.
-   (:pull:`3997:`)
+   (:pull:`3997`)
 
 .. comment
     Whatsnew author names (@github name) in alphabetical order. Note that,