Correct heading for v3.4 release highlights. (#5110)

* Correct heading for v3.4 release highlights.

* Include patch number in release highlights heading.

Co-authored-by: lbdreyer <lbdreyer@users.noreply.github.com>

Co-authored-by: lbdreyer <lbdreyer@users.noreply.github.com>

Author: trexfeathers

docs/src/whatsnew/3.4.rst

@@ -7,7 +7,7 @@ This document explains the changes made to Iris for this release
 (:doc:`View all changes <index>`.)
 
 
-.. dropdown:: :opticon:`report` |iris_version| Release Highlights
+.. dropdown:: :opticon:`report` v3.4.0 Release Highlights
    :container: + shadow
    :title: text-primary text-center font-weight-bold
    :body: bg-light