Fix typo

[numb86]

Please check if the PR fulfills these requirements

• Is this just a doc change? (yes/no)

yes

• If no, then check that:
  • The commit message(s) are descriptive of the changes made
  • The PR contains changes that are focused and differs from other PRs
  • Tests for the changes have been added where needed
  • Docs have been added / updated

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Just fix typo in doc.

[coveralls]

Coverage remained the same at 97.674% when pulling be3509c on numb86:patch-1 into 931b2ec on omnidan:master.

[nmay231]

Thanks! 👍