chore: update black property

pyproject.toml

@@ -18,7 +18,7 @@ contextvars = { version = "^2.4", python = "~3.6" }
 [tool.poetry.dev-dependencies]
 pytest = "^3.0"
 pylint = "^2.4"
-black = {version = "^18.3-alpha.0", allows-prereleases = true}
+black = {version = "^18.3-alpha.0", allow-prereleases = true}
 rope = "^0.14.0"
 isort = "^4.3"