Fix typo in dep name

Author: nicolas-grekas

composer.json

@@ -27,7 +27,7 @@
     },
     "require-dev": {
         "phpunit/phpunit": "^5.7|^6.4|^7.0",
-        "mikey179/vfsStream": "^1.6",
+        "mikey179/vfsstream": "^1.6",
         "doctrine/coding-standard": "^5.0",
         "phpstan/phpstan": "^0.9.2",
         "phpstan/phpstan-strict-rules": "^0.9"