fix(core) Fix the build command typo in README.

Summary:
Some developer called out a issue that sample doesn't build in github #18.

I've tried our README's steps but `pnpm run build:all` kept failing for me. Switching to the build:tgz script that we've been using and validated by QA in our README.

Reviewed By: felixtrz

Differential Revision: D89410409

fbshipit-source-id: d83be2ebea0d8b747e3dc8bdeaf04c35bf1117d7

Author: zjm-meta

README.md

@@ -19,11 +19,11 @@ The **Immersive Web SDK** makes building immersive web experiences as approachab
 # Install dependencies
 pnpm install
 
-# Build all packages
-pnpm run build:all
+# Build and pack all packages
+pnpm run build:tgz
 
 # Run example
-cd examples/locomotion && pnpm run dev
+cd examples/locomotion && pnpm run fresh:dev
 ```
 
 ## Development