kernel: Fix type error when handling kernel ambiguity.

* guix-jupyter-kernel.scm (create-environment): In the case where
several kernels are provided, rename formal parameter from 'file' to
'specs'.

guix-jupyter-kernel.scm

@@ -435,7 +435,7 @@ to KERNEL as a reply to MESSAGE, and return STATE suitably adjusted."
                       (bold "Found " ,(length lst)
                             " kernels in environment "
                             (code ,name) ":")
-                      (ul ,@(map (lambda (file)
+                      (ul ,@(map (lambda (specs)
                                    `(li ,(kernel-specs-display-name specs)))
                                  lst))
                       "Which one should we use?  Please create an "