Fixed typos in paddle2onnx readme

[KevinOfCathay]

No description provided.

[paddle-bot]

Thanks for your contribution!

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ LDOUBLEV
❌ ksana

---

ksana seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[onecatcn]

LGTM

[onecatcn]

@KevinOfCathay thank a lot for you contribution, Please sign the CLA so that I can merge the PR.