Fix sexp_processor gem reference

It's https://rubygems.org/gems/sexp_processor, not https://rubygems.org/gems/sexp

Author: Earlopain

lib/prism/translation/ruby_parser.rb

@@ -4,7 +4,7 @@
 begin
   require "sexp"
 rescue LoadError
-  warn(%q{Error: Unable to load sexp. Add `gem "sexp"` to your Gemfile.})
+  warn(%q{Error: Unable to load sexp. Add `gem "sexp_processor"` to your Gemfile.})
   exit(1)
 end