docs: use the image name make docker builds in the Docker run example

make docker tags the image pixa:latest via script/projectname, so the
Getting Started run line must reference pixa:latest, not pixad:latest.

Model: opus-4-8

README.md

@@ -24,7 +24,7 @@ $EDITOR config.yml   # replace the signing_key placeholder
 
 # or build and run via Docker
 make docker
-docker run -p 8080:8080 -e PIXA_SIGNING_KEY="$(openssl rand -base64 32)" pixad:latest
+docker run -p 8080:8080 -e PIXA_SIGNING_KEY="$(openssl rand -base64 32)" pixa:latest
 ```
 
 A container is configured two ways. The signing key comes from the