Instruction to build semgrep-core from source refers to missing Makefile target

[amfarrell]

I got an error when I ran make build-core as I was following the instructions to build semgrep-core from source.

make: *** No rule to make target `build-core'.  Stop.

This prevented me from following the recommended way to get a semgrep-core binary as I was setting up a development environment to contribute to semgrep-cli. I saw this slack thread, which showed that another person had encountered the same problem with that instruction. I ran git log -S'build-core' -- Makefile on the develop branch of semgrep and I found that the instruction fell out of date as a result of this code change which removed the Makefile target build-core.

I would like help to update the docs with the correct instruction for new contributors to build semgrep-core from source.