docs: add --file-patterns definition format to the error message

[MFTabriz]

Currently, the information for running trivy config --file-patterns xxx is documented in the source code as a comment, and in Skipping Files and Directories page in the documentation [not a place one would normally look for learning about including files!]. But it has not been documented in the Misconfiguration Scanning page, its error message, or output of trivy config --help.

New error message:

> trivy config --file-patterns='bad_format*' ~/my_path

INFO	Misconfiguration scanning is enabled
FATAL	filesystem scan error: scan error: unable to initialize a scanner: unable to initialize a filesystem scanner: analyzer group error: invalid file pattern (bad_format*) expected format: "fileType:regexPattern" e.g. "dockerfile:my_dockerfile_*"

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[knqyf263]

Thanks for your contribution. Could you merge the main branch?

[MFTabriz]

@knqyf263 I rebased it!

[simar7]

thanks lgtm!