Correct spelling of Dockerfile (#319)

Author: tylere

src/docker-existing-dockerfile/devcontainer-template.json

@@ -2,7 +2,7 @@
     "id": "docker-existing-dockerfile",
     "version": "1.3.2",
     "name": "Existing Dockerfile",
-    "description": "Sample illustrating reuse of an existing Dockefile.",
+    "description": "Sample illustrating reuse of an existing Dockerfile.",
     "documentationURL": "https://github.com/devcontainers/templates/tree/main/src/docker-existing-dockerfile",
     "publisher": "Dev Container Spec Maintainers",
     "licenseURL": "https://github.com/devcontainers/templates/blob/main/LICENSE",