docs: fix config variable usage in memory docs (#64469)

Closes vercel/feedback#61864

Author: leerob

docs/02-app/01-building-your-application/06-optimizing/13-memory-usage.mdx

@@ -72,11 +72,11 @@ const nextConfig = {
     config,
     { buildId, dev, isServer, defaultLoaders, nextRuntime, webpack }
   ) => {
-    if (cfg.cache && !dev) {
-      cfg.cache = Object.freeze({
+    if (config.cache && !dev) {
+      config.cache = Object.freeze({
         type: 'memory',
       })
-      cfg.cache.maxMemoryGenerations = 0
+      config.cache.maxMemoryGenerations = 0
     }
     // Important: return the modified config
     return config