Fix templating error

flaskbb/templates/email/reset_password.txt

@@ -1,4 +1,4 @@
-{{ set link = url_for('auth.reset_password', token=token, _external=True) }}
+{% set link = url_for('auth.reset_password', token=token, _external=True) %}
 
 {% trans user=user.username, link=link %}Dear {{ user }},