Fix typo in coverage testenv

Author: domdfcoding

tox.ini

@@ -144,7 +144,7 @@ changedir = {toxinidir}
 deps = coverage>=5
 commands =
     /bin/bash -c "rm -rf htmlcov"
-    coverage html --rc-file cov-report.ini
+    coverage html --rcfile cov-report.ini
     /bin/bash -c "DISPLAY=:0 firefox 'htmlcov/index.html'"
 
 [coverge:report]