Fix deprecation warning

Rakefile:15: warning: rdoc/task was loaded from the standard library, but is not part of the default gems starting from Ruby 3.5.0.

Author: stympy

honeybadger.gemspec

@@ -35,6 +35,7 @@ Gem::Specification.new do |s|
 
   s.executables << "honeybadger"
 
+  s.add_development_dependency "rdoc"
   s.add_dependency "logger"
   s.add_dependency "ostruct"
 end