fix: replace calls to `File.exists?` with `File.exist?`

This prints warnings about being deprecated, so replace it with the
call that the deprecated function wraps.

See: https://github.com/ruby/ruby/blob/ruby_2_7/file.c#L1752-L1781

lib/docs/storage/file_store.rb

@@ -30,7 +30,7 @@ module Docs
     end
 
     def file_exist?(path)
-      File.exists?(path)
+      File.exist?(path)
     end
 
     def file_mtime(path)
@@ -46,7 +46,7 @@ module Docs
         next if file == path
         Find.prune if File.basename(file)[0] == '.'
         yield file
-        Find.prune unless File.exists?(file)
+        Find.prune unless File.exist?(file)
       end
     end
   end

lib/tasks/docs.thor

@@ -180,7 +180,7 @@ class DocsCLI < Thor
         return
       end
 
-      unless File.exists?(File.join(Docs.store_path, "#{doc.path}.tar.gz"))
+      unless File.exist?(File.join(Docs.store_path, "#{doc.path}.tar.gz"))
         puts "ERROR: package for '#{doc.slug}' documentation not found. Run 'thor docs:package #{doc.slug}' to create it."
         return
       end