fix: mention "3" as a valid value for "resolver" field in error message (#15215)

<!--
Thanks for submitting a pull request 🎉! Here are some tips for you:

* If this is your first contribution, read "Cargo Contribution Guide"
first:
  https://doc.crates.io/contrib/
* Run `cargo fmt --all` to format your code changes.
* Small commits and pull requests are always preferable and easy to
review.
* If your idea is large and needs feedback from the community, read how:
  https://doc.crates.io/contrib/process/#working-on-large-features
* Cargo takes care of compatibility. Read our design principles:
  https://doc.crates.io/contrib/design.html
* When changing help text of cargo commands, follow the steps to
generate docs:

https://github.com/rust-lang/cargo/tree/master/src/doc#building-the-man-pages
* If your PR is not finished, set it as "draft" PR or add "WIP" in its
title.
* It's ok to use the CI resources to test your PR, but please don't
abuse them.

### What does this PR try to resolve?

Explain the motivation behind this change.
A clear overview along with an in-depth explanation are helpful.

You can use `Fixes #<issue number>` to associate this PR to an existing
issue.

### How should we test and review this PR?

Demonstrate how you test this change and guide reviewers through your
PR.
With a smooth review process, a pull request usually gets reviewed
quicker.

If you don't know how to write and run your tests, please read the
guide:
https://doc.crates.io/contrib/tests

### Additional information

Other information you want to mention in this PR, such as prior arts,
future extensions, an unresolved problem, or a TODO list.
-->

### What does this PR try to resolve?

Just a small update to the error message for the resolver field, which
now accepts "3" as a valid value.

### How should we test and review this PR?

### Additional information

Author: epage

src/cargo/core/resolver/types.rs

@@ -122,7 +122,7 @@ impl ResolveBehavior {
             "2" => Ok(ResolveBehavior::V2),
             "3" => Ok(ResolveBehavior::V3),
             s => anyhow::bail!(
-                "`resolver` setting `{}` is not valid, valid options are \"1\" or \"2\"",
+                "`resolver` setting `{}` is not valid, valid options are \"1\", \"2\" or \"3\"",
                 s
             ),
         }

tests/testsuite/features2.rs

@@ -1391,7 +1391,7 @@ fn resolver_bad_setting() {
 [ERROR] failed to parse manifest at `[ROOT]/foo/Cargo.toml`
 
 Caused by:
-  `resolver` setting `foo` is not valid, valid options are "1" or "2"
+  `resolver` setting `foo` is not valid, valid options are "1", "2" or "3"
 
 "#]])
         .run();