Fix example usage of COMPOSE_ENV_FILES (#23253)

## Description
The example results in the second file being a command to be executed
and not part of the environment variable's value.

Quoting the value would address it as well, but thought it far more
likely the space was an accident rather than trying to show an example
of files containing spaces.

Author: rrjjvv

content/manuals/compose/how-tos/environment-variables/envvars.md

@@ -119,7 +119,7 @@ Specifies which environment files Compose should use if `--env-file` isn't used.
 When using multiple environment files, use a comma as a separator. For example: 
 
 ```console
-COMPOSE_ENV_FILES=.env.envfile1, .env.envfile2
+COMPOSE_ENV_FILES=.env.envfile1,.env.envfile2
 ```
 
 If `COMPOSE_ENV_FILES` is not set, and you don't provide `--env-file` in the CLI, Docker Compose uses the default behavior, which is to look for an `.env` file in the project directory.