Fix checkdoc problems with new custom file

core/prelude-custom.el

@@ -44,7 +44,7 @@
   :group 'prelude)
 
 (defcustom prelude-guru t
-  "Non-nil values enable guru-mode."
+  "Non-nil values enable `guru-mode'."
   :type 'boolean
   :group 'prelude)
 
@@ -55,7 +55,7 @@
 
 (defcustom prelude-clean-whitespace-on-save t
   "Cleanup whitespace from file before it's saved.
-Will only occur if prelude-whitespace is also enabled."
+Will only occur if `prelude-whitespace' is also enabled."
   :type 'boolean
   :group 'prelude)