use :utf8 symbol directly in external-format

sbcl can use :utf8 as a direct symbol also

src/config.lisp

 (defun load-config (&optional (repo-dir ""))
   "Find and load the coleslaw configuration from .coleslawrc. REPO-DIR will be
 preferred over the home directory if provided."
-  (with-open-file (in (discover-config-path repo-dir) :external-format '(:utf-8))
+  (with-open-file (in (discover-config-path repo-dir) :external-format :utf-8)
     (let ((config-form (read in)))
       (setf *config* (construct 'blog config-form)
             (repo *config*) repo-dir)))

src/content.lisp

                                       (file-position stream)))))
              (read-sequence seq stream)
              (remove #\Nul seq))))
-    (with-open-file (in file :external-format '(:utf-8))
+    (with-open-file (in file :external-format :utf-8)
       (let ((metadata (parse-metadata in))
             (content (slurp-remainder in))
             (filepath (enough-namestring file (repo *config*))))

src/util.lisp

                    :direction :output
                    :if-exists :supersede
                    :if-does-not-exist :create
-                   :external-format '(:utf-8))
+                   :external-format :utf-8)
     (write text :stream out :escape nil)))
 
 (defun get-updated-files (&optional (revision *last-revision*))