Documentation fix: Execute trailing slash stripping function, rather …

…than assign it. (#9924)

The trailing slash stripping plugin [executes the function](https://github.com/gatsbyjs/gatsby/blob/f9714a5646523bc728586e3692190507ad68e6c1/packages/gatsby-plugin-remove-trailing-slashes/src/gatsby-node.js#L9) rather than just assigning it, so I'd imagine that's what the example code should be doing too.

docs/docs/creating-and-modifying-pages.md

@@ -114,14 +114,16 @@ _Note: There's also a plugin that will remove all trailing slashes from pages au
 [gatsby-plugin-remove-trailing-slashes](/packages/gatsby-plugin-remove-trailing-slashes/)_.
 
 ```javascript:title=gatsby-node.js
+// Replacing '/' would result in empty string which is invalid
+const replacePath = path => (path === `/` ? path : path.replace(/\/$/, ``))
 // Implement the Gatsby API “onCreatePage”. This is
 // called after every page is created.
 exports.onCreatePage = ({ page, actions }) => {
   const { createPage, deletePage } = actions
   return new Promise(resolve => {
     const oldPage = Object.assign({}, page)
     // Remove trailing slash unless page is /
-    page.path = _path => (_path === `/` ? _path : _path.replace(/\/$/, ``))
+    page.path = replacePath(page.path)
     if (page.path !== oldPage.path) {
       // Replace new page with old page
       deletePage(oldPage)