Docs: fix env_file example

Thanks-to: @aanand
Signed-off-by: Laurent Arnoud <laurent@spkdev.net>

docs/yml.md

@@ -172,8 +172,12 @@ env_file:
   - /opt/secrets.env
 ```
 
+Compose expects each line in an env file to be in `VAR=VAL` format. Lines
+beginning with `#` (i.e. comments) are ignored, as are blank lines.
+
 ```
-RACK_ENV: development
+# Set Rails/Rack environment
+RACK_ENV=development
 ```
 
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