fix SPA handler in README.md (#733)

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## What type of PR is this? (check all applicable)

- [ ] Refactor
- [ ] Feature
- [ ] Bug Fix
- [ ] Optimization
- [X] Documentation Update
- [ ] Go Version Update
- [ ] Dependency Update

## Description

Changed the SPA handler example in README.md in two areas. First, made
sure to actually include the requested path in the call to
`filepath.Join`. Secondly, if the requested path hits a directory, I
think it would be beneficial to also serve the `indexPath` file, and not
list the directory contents. I also edited the comments in the
`README.md` file accordingly.

## Related Tickets & Documents

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- Closes #

## Added/updated tests?

- [ ] Yes
- [X] No, and this is why: I only changed the `README.md`, if any tests
are necessary please let me know
- [ ] I need help with writing tests

## Run verifications and test

- [ ] `make verify` is passing
- [ ] `make test` is passing

README.md

@@ -247,23 +247,25 @@ type spaHandler struct {
 // file located at the index path on the SPA handler will be served. This
 // is suitable behavior for serving an SPA (single page application).
 func (h spaHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-    // Join internally call path.Clean to prevent directory traversal
-    path := filepath.Join(h.staticPath, path)
+	// Join internally call path.Clean to prevent directory traversal
+	path := filepath.Join(h.staticPath, r.URL.Path)
 
-    // check whether a file exists at the given path
-	_, err := os.Stat(path)
-	if os.IsNotExist(err) {
-		// file does not exist, serve index.html
+	// check whether a file exists or is a directory at the given path
+	fi, err := os.Stat(path)
+	if os.IsNotExist(err) || fi.IsDir() {
+		// file does not exist or path is a directory, serve index.html
 		http.ServeFile(w, r, filepath.Join(h.staticPath, h.indexPath))
 		return
-	} else if err != nil {
-        // if we got an error (that wasn't that the file doesn't exist) stating the
-        // file, return a 500 internal server error and stop
+	}
+
+	if err != nil {
+		// if we got an error (that wasn't that the file doesn't exist) stating the
+		// file, return a 500 internal server error and stop
 		http.Error(w, err.Error(), http.StatusInternalServerError)
-		return
+        return
 	}
 
-    // otherwise, use http.FileServer to serve the static dir
+	// otherwise, use http.FileServer to serve the static file
 	http.FileServer(http.Dir(h.staticPath)).ServeHTTP(w, r)
 }