Fix filename in contribute-docs.rst

[dibyaaaaax]

There was a small error in the filename in the "how-to" instruction page. The error is fixed.

Following the instructions word by word resulted in an 'no such file or directory' error. Eliminates the prospect of such error.

PR Checklist:

• [x] All new features have been tested
• [ ] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct

[danyeaw]

Thanks!