address suggested changes

Signed-off-by: Stephanie <yangcao@redhat.com>

Author: yangcao77

pkg/validation/validation-rule.md

@@ -34,7 +34,7 @@ Common rules for all components types:
 
 #### Container component 
 1. the container components must reference a valid volume component if it uses volume mounts, and the volume components are unique
-2. `PROJECT_SOURCE` or `PROJECTS_ROOT` are reserved env, cannot be defined again in env
+2. `PROJECT_SOURCE` or `PROJECTS_ROOT` are reserved environment variables defined under env, cannot be defined again in `env`
 
 #### Plugin Component
 - Commands in plugins components share the same commands validation rules as listed above. Validation occurs after overriding and merging, in flattened devfile