Use #sub method over #match & fix regex

It's better if we use #sub over #match + #delete_prefix.
Also, add "^" to regex, thereby fixing false-positive
cases (for eg: "/lib/foo/bar").

lib/rubocop/cop/packaging/require_with_relative_path.rb

@@ -73,8 +73,7 @@ def on_send(node)
         # Called from on_send, this method helps to replace
         # the "bad" require call with the "good" one.
         def good_require_call
-          bad_part = @str.match(%r{.*/lib/}).to_s
-          good_call = @str.delete_prefix(bad_part)
+          good_call = @str.sub(%r{^.*/lib/}, "")
           %(require "#{good_call}")
         end