http-client: '%http-cache-ttl' is really a parameter.

Fixes a typo in commit 739ab68b that made it a procedure returning a
parameter.

* guix/http-client.scm (%http-cache-ttl): Turn into a parameter.

guix/http-client.scm

@@ -241,7 +241,7 @@ (define* (http-fetch uri #:key port (text? #f) (buffered? #t))
 ;;; Caching.
 ;;;
 
-(define (%http-cache-ttl)
+(define %http-cache-ttl
   ;; Time-to-live in seconds of the HTTP cache of in ~/.cache/guix.
   (make-parameter
    (* 3600 (or (and=> (getenv "GUIX_HTTP_CACHE_TTL")