gnu: r-tractor-base: Use HTTPS home page.

* gnu/packages/cran.scm (r-tractor-base)[home-page]: Use HTTPS.

gnu/packages/cran.scm

@@ -10317,7 +10317,7 @@ (define-public r-tractor-base
        ("r-reportr" ,r-reportr)
        ("r-rnifti" ,r-rnifti)
        ("r-shades" ,r-shades)))
-    (home-page "http://www.tractor-mri.org.uk")
+    (home-page "https://www.tractor-mri.org.uk")
     (synopsis "Read, manipulate and visualize magnetic resonance images")
     (description
      "This package provides functions for working with magnetic resonance