Update upgrade-guide.md

[bachinblack]

Please correct me if I'm wrong :slightly_smiling_face:

• If one wants to keep using doctrine inflector, they should set keep_legacy_inflector to true, not false.
• The link on validating a delete operation seemed broken.

I'm sorry I'm fairly new to making PRs, not sure if I should make a PR to main or to a different version.

[bachinblack]

Hey ! I'm sorry, I don't understand why the pipeline's failing. Could anyone help me figure out the problem ?

[pieterocp]

@bachinblack without a contributor / a pipe re-run, the actions logs are now missing. A re-run by a contributor is required to get to the bottom of this.