docs: Handle slash trimmed path in example server

[blmhemu]

The tusd handler works on slash trimmed paths i.e /files. Making the example server do the same.

Usecase:
Just wasted a couple of hours debugging this behaviour using the example server.

[Acconut]

Thank you very much! The comment above the call to http.Handle also just adds to the confusion, making this a great improvement.

Just as a reference for others: This mirrors the logic from the tusd CLI:

tusd/cmd/tusd/cli/serve.go

Lines 94 to 100 in 15f05c0

	// ... and register a route with and without the trailing slash, so we can
	// handle uploads for /files/ and /files, for example.
	basepathWithoutSlash := strings.TrimSuffix(basepath, "/")
	basepathWithSlash := basepathWithoutSlash + "/"
	mux.Handle(basepathWithSlash, http.StripPrefix(basepathWithSlash, handler))
	mux.Handle(basepathWithoutSlash, http.StripPrefix(basepathWithoutSlash, handler))