docs: fix spelling and grammar (#13862)

Fixed various spelling errors and grammatical issues across multiple .mdx documentation files.

www/apps/resources/app/storefront-development/checkout/payment/page.mdx

@@ -59,7 +59,7 @@ export const highlights = [
   ["57", "activePaymentSession", "The active session is the first in the payment collection's sessions."],
   ["62", "", "Test which payment provider is chosen based on the prefix of the provider ID."],
   ["63", `"pp_stripe_"`, "Check if the chosen provider is Stripe."],
-  ["71", `"pp_system_default"`, "Check if the chosen provider is the default systen provider."],
+  ["71", `"pp_system_default"`, "Check if the chosen provider is the default system provider."],
   ["77", "default", "Handle unrecognized providers."],
   ["112", "getPaymentUi", "If a provider is chosen, render its UI."]
 ]
@@ -195,7 +195,7 @@ export const fetchHighlights = [
   ["32", "activePaymentSession", "The active session is the first in the payment collection's sessions."],
   ["38", "", "Test which payment provider is chosen based on the prefix of the provider ID."],
   ["39", `"pp_stripe_"`, "Check if the chosen provider is Stripe."],
-  ["43", `"pp_system_default"`, "Check if the chosen provider is the default systen provider."],
+  ["43", `"pp_system_default"`, "Check if the chosen provider is the default system provider."],
   ["45", "default", "Handle unrecognized providers."],
   ["52", "handlePayment", "The function that handles the payment process using the above functions."]
 ]

www/apps/resources/app/storefront-development/checkout/payment/stripe/page.mdx

@@ -185,7 +185,7 @@ const StripeForm = ({
       sdk.store.cart.complete(cart.id)
       .then((data) => {
         if (data.type === "cart" && data.cart) {
-          // an error occured
+          // an error occurred
           console.error(data.error)
         } else if (data.type === "order" && data.order) {
           // TODO redirect to order success page