Update conflicting docs for default number of jobs

[adamcohen2]

This PR removes the absent=<number-of-cores-deteced> from the docs, since this value is variable and depends on the number of cores of the machine where semgrep is run. This leads to conflicting information in the docs, because the description of the --jobs parameter explains that it defaults to the number of cores detected on the system, whereas the absent=4 value is hardcoded, which is confusing.

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