Simple code change to eliminate deprecated warning (See issue #99)

[driverjb]

Checklist

• run npm run test and npm run benchmark
  • npm run benchmark does not exist
• tests and/or benchmarks are included

λ npm run test

> @fastify/accepts-serializer@5.2.0 test
> npm run test:unit && npm run test:typescript


> @fastify/accepts-serializer@5.2.0 test:unit
> tap

​ PASS ​ test/test.js 55 OK 227.309ms
​


  🌈 SUMMARY RESULTS 🌈

​
Suites:   ​1 passed​, ​1 of 1 completed​
Asserts:  ​​​55 passed​, ​of 55​
​Time:​   ​6s​
----------------------|---------|----------|---------|---------|-------------------
File                  | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------------------|---------|----------|---------|---------|-------------------
All files             |     100 |      100 |     100 |     100 |
 SerializerManager.js |     100 |      100 |     100 |     100 |
 index.js             |     100 |      100 |     100 |     100 |
----------------------|---------|----------|---------|---------|-------------------

> @fastify/accepts-serializer@5.2.0 test:typescript
> tsd

• documentation is changed or added
  • Not applicable in this change
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[climba03003]

Please do not change the version here.
It should be handled by the releaser.

[driverjb]

Do you want me to submit a new PR or can we change it here directly?

[driverjb]

I've modified the version back to the original value of 5.2.0

[mcollina]

lgtm