Fixed small typo in README code example

[themasch]

Small typo in a code example in the README, resulting in syntax error.

Description

Removed an additional [ in one code example, where it caused a parse error

Motivation and context

Confused me for a second or two, since I though I missed new type of syntax getting added to attributes. But as far as I (and 3v4l.org) know, its a parse error like this. Fixiting this prevents confusion ;)

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] My pull request addresses exactly one patch/feature.
• [x] I have created a branch for this patch/feature.
• [x] Each individual commit in the pull request is meaningful.
• [ ] I have added tests to cover my changes. // doe that apply here?
• [x] If my change requires a change to the documentation, I have updated it accordingly.

If you're unsure about any of these, don't hesitate to ask. We're here to help!

[Crell]

Oops! You are correct. Thanks.