Fix lexer language in upgrade guide for Field validation in blocks ex…

…ample.

docs/source/upgrade-guide/index.md

@@ -395,7 +395,7 @@ If you shadowed the module {file}`packages/volto/src/helpers/FormValidation/Form
 This prop must be assigned with the new prop passed down from the blocks engine `blocksErrors`.
 If not passed down, the block can't display any field validation error.
 
-```tsx
+```jsx
 // More component code above here
 
   const {