Update README.md

Fix example based on discussion in anothrNick#215

README.md

@@ -35,7 +35,7 @@ jobs:
 ```
 
 ```yaml
-# example 2: on merge to master
+# example 2: on merge to master from pull request (recommended)
 name: Bump version
 on:
   pull_request:
@@ -45,12 +45,13 @@ on:
       - master
 jobs:
   build:
+    if: github.event.pull_request.merged == true
     runs-on: ubuntu-22.04
     steps:
     - uses: actions/checkout@v3
       with:
-        ref: ${{ github.event.pull_request.head.sha }}
-        fetch-depth: '0'
+        ref: ${{ github.event.pull_request.merge_commit_sha }}
+        fetch-depth: "0"
 
     - name: Bump version and push tag
       uses: anothrNick/github-tag-action@v1