change npx sst dev to pnpm sst dev

[yyaskriloff]

Change npx to pnpm to be consistent with the documentation earlier on the same page. In addition, every section named Deploy Our Changes in the following chapter (BUILDING A SERVERLESS API) references this command using pnpm and not npx.

[yyaskriloff]

Also when going through the guide the first time the dev server is started is using pnpm. Create A Hello World Api and really every other time is using pnpm like pnpm sst remove API

[jayair]

Thanks!