tox: fix typo in lint source dir expansion

Author: xrmx

tox.ini

@@ -1198,8 +1198,8 @@ deps =
   -r dev-requirements.txt
 
 commands =
-  black --config {toxinidir}/pyproject.toml {{toxinidir}} --diff --check
-  isort --settings-path {toxinidir}/.isort.cfg {{toxinidir}} --diff --check-only
+  black --config {toxinidir}/pyproject.toml {toxinidir} --diff --check
+  isort --settings-path {toxinidir}/.isort.cfg {toxinidir} --diff --check-only
   flake8 --config {toxinidir}/.flake8 {toxinidir}
 
 [testenv:spellcheck]