Rename pretty_print alias to summary

This stops clobbering the `Object#pretty_print` mixin from the `pp` gem,
which is needed to correctly print a `Checkable` as a return value in IRB.

If you used `pretty_print`, use `to_s_pretty` or `summary` instead.

Author: TooManyBees

lib/github-pages-health-check/checkable.rb

@@ -48,10 +48,10 @@ def to_s
         printer.simple_string
       end
 
-      def to_s_pretty(*)
+      def to_s_pretty
         printer.pretty_print
       end
-      alias pretty_print to_s_pretty
+      alias summary to_s_pretty
 
       private