docs: small typo fix (medusajs#12843)

Author: shahednasser

www/apps/book/app/learn/fundamentals/modules/options/page.mdx

@@ -123,7 +123,7 @@ export default async function helloWorldLoader({
 
 ## Validate Module Options
 
-If you expect a certain option and want to throw an error if it's not provided or isn't valid, it's recommended to perform the validation in a loader. The module's service is only instantiated when it's used, whereas the loader runs the when the Medusa application starts.
+If you expect a certain option and want to throw an error if it's not provided or isn't valid, it's recommended to perform the validation in a loader. The module's service is only instantiated when it's used, whereas the loader runs when the Medusa application starts.
 
 So, by performing the validation in the loader, you ensure you can throw an error at an early point, rather than when the module is used.