docs: fix instructions

readme.md

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 ## Prerequisites
 
 - Install the `forkme` utility from https://tangled.org/me.webbeef.org/forkme
-- Run `forkme update`
-- Run `forkme apply`
+- Run `forkme init`
 - From the `source` directory, run `./mach bootstrap` to setup Servo build dependencies.
 
 ## Desktop builds