fix: correct grammar in environment variables section

Change "are supported" to "is supported" since the subject is "list" (singular).

Fixes #23616

Author: Andrew Hopp

data/engine-cli/docker.yaml

@@ -29,7 +29,7 @@ long: |-
 
     ### Environment variables
 
-    The following list of environment variables are supported by the `docker` command
+    The following list of environment variables is supported by the `docker` command
     line:
 
     | Variable                      | Description                                                                                                                                                                                                                                                       |