docs(gatsby-plugin-offline): Change runtimeCaching handlers (#21494)

Change docs to be compliant with the code: https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-offline/src/gatsby-node.js#L132

And using current example from docs prints following error to console:
```
Specifying 'cacheFirst'' in a 'runtimeCaching[].handler' option is deprecated. Please update your config to use 'CacheFirst'
```

packages/gatsby-plugin-offline/README.md

@@ -143,22 +143,22 @@ const options = {
       // Use cacheFirst since these don't need to be revalidated (same RegExp
       // and same reason as above)
       urlPattern: /(\.js$|\.css$|static\/)/,
-      handler: `cacheFirst`,
+      handler: `CacheFirst`,
     },
     {
       // page-data.json files are not content hashed
       urlPattern: /^https?:.*\page-data\/.*\/page-data\.json/,
-      handler: `networkFirst`,
+      handler: `NetworkFirst`,
     },
     {
       // Add runtime caching of various other page resources
       urlPattern: /^https?:.*\.(png|jpg|jpeg|webp|svg|gif|tiff|js|woff|woff2|json|css)$/,
-      handler: `staleWhileRevalidate`,
+      handler: `StaleWhileRevalidate`,
     },
     {
       // Google Fonts CSS (doesn't end in .css so we need to specify it)
       urlPattern: /^https?:\/\/fonts\.googleapis\.com\/css/,
-      handler: `staleWhileRevalidate`,
+      handler: `StaleWhileRevalidate`,
     },
   ],
   skipWaiting: true,