Docs: Fix the block metadata details for the category field

As discussed in #35998 (comment) the category field is no longer mandatory. The documentation wasn't fully updated even though there was already the following sentence included at the time it became optional:

> An implementation should expect and tolerate unknown categories, providing some reasonable fallback behavior (e.g. a "text" category).

docs/reference-guides/block-api/block-metadata.md

@@ -176,7 +176,7 @@ This is the display title for your block, which can be translated with our trans
 ### Category
 
 -   Type: `string`
--   Required
+-   Optional
 -   Localized: No
 -   Property: `category`