doc: add missing ' in README.md codeblock (#345)

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

comps/llms/text-generation/vllm/README.md

@@ -123,5 +123,5 @@ bash launch_microservice.sh
 curl http://${your_ip}:9000/v1/chat/completions \
   -X POST \
   -d '{"query":"What is Deep Learning?","max_new_tokens":17,"top_p":0.95,"temperature":0.01,"streaming":false}' \
-  -H 'Content-Type: application/json
+  -H 'Content-Type: application/json'
 ```