Fix up incorrect code snippet

[poltak]

• Something fairly small that I noticed earlier when I tried to play with plugins, although not entirely sure if it's something you intended.
• Custom operations afforded by plugins seem to be only available on the backend, rather than the storex instance itself.

[ShishKabab]

Yeah, just fixed that in a commit yesterday when I added support for middleware: https://github.com/WorldBrain/storex/commit/fc5de2abaf593943e2a636c864424924817cfe22#diff-2d03800ce419d2ac7934b2b9841fc46bR80

Thanks for noticing! Still need to make this into a 0.2.0 release though...

[ShishKabab]

Just released ;)