fix(baggage-span-processor): cannot find module (#2260)

Fixes the following error:
```
Error: Cannot find module '/Users/long/Repos/wise-rock/node_modules/@opentelemetry/baggage-span-processor/build/src/index.ts'. Please verify that the package.json has a valid "main" entry
    at tryPackage (node:internal/modules/cjs/loader:415:19)
...
```

Author: longility

packages/baggage-span-processor/package.json

@@ -2,7 +2,7 @@
   "name": "@opentelemetry/baggage-span-processor",
   "version": "0.3.0",
   "description": "OpenTelemetry Baggage Span Processor for Node.js",
-  "main": "build/src/index.ts",
+  "main": "build/src/index.js",
   "types": "build/src/index.d.ts",
   "repository": "open-telemetry/opentelemetry-js-contrib",
   "scripts": {