docs: use buildkite plugin slug

Author: lox

.buildkite/pipeline.yml

@@ -6,7 +6,7 @@ steps:
   - label: ":lint-roller: Lint"
     plugins:
       - plugin-linter#v3.0.0:
-          id: buildkite-plugins/mise
+          id: buildkite/mise
 
   - label: ":mise: Smoke test"
     command:

README.md

@@ -15,7 +15,7 @@ This plugin is intentionally small:
 steps:
   - label: ":wrench: Test"
     plugins:
-      - buildkite-plugins/mise#v1.1.0:
+      - buildkite/mise#v1.1.0:
           version: 2026.2.11
     command: go test ./...
 ```
@@ -26,7 +26,7 @@ steps:
 steps:
   - label: ":wrench: Test backend"
     plugins:
-      - buildkite-plugins/mise#v1.1.0:
+      - buildkite/mise#v1.1.0:
           dir: backend
     command: go test ./...
 ```
@@ -39,7 +39,7 @@ cache: ".buildkite/cache-volume"
 steps:
   - label: ":wrench: Test"
     plugins:
-      - buildkite-plugins/mise#v1.1.0: ~
+      - buildkite/mise#v1.1.0: ~
     command: go test ./...
 ```
 
@@ -67,7 +67,7 @@ Run plugin checks locally:
 
 ```bash
 mise install
-docker run --rm -v "$PWD:/plugin" -w /plugin buildkite/plugin-linter --id buildkite-plugins/mise --path /plugin
+docker run --rm -v "$PWD:/plugin" -w /plugin buildkite/plugin-linter --id buildkite/mise --path /plugin
 docker run --rm -v "$PWD:/plugin" -w /plugin buildkite/plugin-tester
 "$(mise where shellcheck@0.11.0)/shellcheck-v0.11.0/shellcheck" hooks/pre-command tests/pre-command.bats
 ```