chore: fix examples (noir-lang#5357)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

Some minor fixes to the `examples` folder I noticed while debugging a
user issue.


## Additional Context



## Documentation\*

Check one:
- [ ] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [ ] I have tested the changes locally.
- [ ] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

examples/.gitignore

@@ -0,0 +1,2 @@
+target
+proofs

examples/codegen_verifier/.gitignore

@@ -1,4 +1,3 @@
 out
 cache
-target
 src/contract.sol

examples/prove_and_verify/prove_and_verify.sh

@@ -11,4 +11,4 @@ $BACKEND prove -b ./target/hello_world.json -w ./target/witness.gz
 
 # TODO: backend should automatically generate vk if necessary.
 $BACKEND write_vk -b ./target/hello_world.json
-$BACKEND verify -v ./target/vk -p ./proofs/proof
+$BACKEND verify -k ./target/vk -p ./proofs/proof