Fix NameError: uninitialized constant Roo::Base::TEMP_PREFIX

Fix reference to deleted `TEMP_PREFIX` constant.  The constant was
replaced with a class method, but in this context we would be calling an
instance method, not a class method, which is why we are getting the
uninitialized constant error.

lib/roo/base.rb

@@ -545,7 +545,7 @@ def find_basename(filename)
 
   def make_tmpdir(prefix = nil, root = nil, &block)
     warn '[DEPRECATION] extend Roo::Tempdir and use its .make_tempdir instead'
-    prefix = "#{TEMP_PREFIX}#{prefix}"
+    prefix = "#{Roo::TEMP_PREFIX}#{prefix}"
     root ||= ENV['ROO_TMP']
 
     if block_given?