update README with latest Bitbucket Pipelines YAML example (#24)

| 🚥 Related to readmeio/micro#370 |
| :------------------- |

## 🧰 Changes

Update README to match the changes to the sample
`bitbucket-pipelines.yml` with the [snippet shown during
onboarding](https://github.com/readmeio/micro/pull/370/files#diff-4462f1adfcf85c8b89018cc216efb2b8893b6cdb1ff15d8547a356a32a340604R9-R10)
on [micro.readme.com](https://micro.readme.com).


## 🧬 QA & Testing

1. Verify the pipeline snippet changes match.
2. [Validate the pipeline
YAML](https://bitbucket-pipelines.prod.public.atl-paas.net/validator).

README.md

@@ -13,7 +13,7 @@ Send your OAS files to [ReadMe Micro](https://micro.readme.com/)
 ```yaml
 name: ReadMe Micro
 
-# Run workflow for every push to the `main` branch
+# Run workflow to sync OpenAPI files for every push to the `main` branch
 on:
   push:
     branches:
@@ -43,10 +43,11 @@ npx @readme/[email protected] './*{yaml,yml,json}' --key=$README_MICRO_SECRET
 image: node:18
 
 pipelines:
-  default:
-    - parallel:
-        - step:
-            name: ReadMe Micro
-            script:
-              - npx @readme/[email protected] './*{yaml,yml,json}' --key=$README_MICRO_SECRET
+  branches:
+    # Run Pipeline to sync OpenAPI files for every push to the `main` branch
+    main:
+      - step:
+          name: ReadMe Micro
+          script:
+            - npx @readme/[email protected] './*{yaml,yml,json}' --key=$README_MICRO_SECRET
 ```