Use correct quotation for product category create example in PHP

[vedanshujain]

As reported on Twitter (with user handle JJAllenTech) we have a typo error in the example for Product category creation which breaks the syntax highlighting.

We would want to fix this here https://github.com/woocommerce/woocommerce-rest-api-docs/blob/trunk/source/includes/wp-api-v3/_product-categories.md?plain=1#L79