gnu: coq-bignums: Fix comment spacing.

* gnu/packages/coq.scm (coq-bignums)[arguments]: Fix comment spacing after
  disabling test phase.

gnu/packages/coq.scm

@@ -437,7 +437,7 @@ (define-public coq-bignums
     (inputs
      `(("camlp5" ,camlp5)))
     (arguments
-     `(#:tests? #f; No test target
+     `(#:tests? #f ; No test target.
        #:make-flags
        (list (string-append "COQLIBINSTALL=" (assoc-ref %outputs "out")
                             "/lib/coq/user-contrib"))