fix reproducible-builds.org/{howto,spec} redirects

don't redirects /howto and /specs and anything below

before for example the following links were wrongly redirected:
https://reproducible-builds.org/howto (without /)
https://reproducible-builds.org/specs (without /)
https://reproducible-builds.org/specs/source-date-epoch-spec/

hosts/jenkins/etc/apache2/sites-available/jenkins.debian.net

@@ -299,8 +299,8 @@ Use common-debian-service-https-redirect reproducible-builds.org
 
 	# redirects reproducible-builds.org to https://reproducible.debian.net except for /specs/ and /howto/
 	RewriteCond %{HTTP_HOST} reproducible-builds\.org
-	RewriteCond %{REQUEST_URI} !^/specs/$
-	RewriteCond %{REQUEST_URI} !^/howto/$
+	RewriteCond %{REQUEST_URI} !^/specs($|/)
+	RewriteCond %{REQUEST_URI} !^/howto($|/)
 	RewriteRule ^/?(.*) https://reproducible.debian.net/$1 [R=301,L]
 
 	<Proxy *>