docs: update docs for correct command to export RENDER_GROUP_ID (#271)

Author: gooishin

usecases/ai/digital-avatar/README.md

@@ -71,7 +71,7 @@ docker compose build
 
 ### Start Docker container
 ```bash
-RENDER_GROUP_ID=$(getent group render | cut -d: -f3)
+export RENDER_GROUP_ID=$(getent group render | cut -d: -f3)
 docker compose up -d
 ```
 

usecases/ai/microservices/text-generation/vllm/README.md

@@ -46,7 +46,7 @@ docker run -it --rm \
 
 * **GPU**
 ```bash
-RENDER_GROUP_ID=$(getent group render | cut -d: -f3)
+export RENDER_GROUP_ID=$(getent group render | cut -d: -f3)
 docker run -it --rm \
     --group-add $RENDER_GROUP_ID \
     --device /dev/dri:/dev/dri \

usecases/ai/openwebui-ollama/README.md

@@ -60,7 +60,7 @@ docker compose build
 
 ### 2. Start the Docker Container
 ```
-RENDER_GROUP_ID=$(getent group render | cut -d: -f3)
+export RENDER_GROUP_ID=$(getent group render | cut -d: -f3)
 docker compose up -d
 ```