Update edf.md

Author: gwangmu

docs/software/container-engine/edf.md

@@ -185,7 +185,7 @@ Environment variable expansion allows for dynamic substitution of environment va
 
  * *Syntax*. Use `${VAR}` to reference an environment variable `VAR`. The variable's value is resolved from the combined environment, which includes variables defined in the host and the container image, the later taking precedence.
  * *Scope*. Variable expansion is supported across all EDF parameters. This includes EDF’s parameters like mounts, workdir, image, etc. For example, `${SCRATCH}` can be used in mounts to reference a directory path.
- * *Undefined Variables*. Referencing an undefined variable results in an error. To safely handle undefined variables, you can use the syntax ${VAR:-}, which evaluates to an empty string if VAR is undefined.
+ * *Undefined Variables*. Referencing an undefined variable results in an error. To safely handle undefined variables, you can use the syntax `${VAR:-}`, which evaluates to an empty string if VAR is undefined.
  * *Preventing Expansion*. To prevent expansion, use double dollar signs $$. For example, `$${VAR}` will render as the literal string `${VAR}`.
  * *Limitations*
     * Variables defined within the `[env]` EDF table cannot reference other entries from `[env]` tables in the same or other EDF files (e.g. the ones entered as base environments). Therefore, only environment variables from the host can be referenced.