config: use --value instead of value-pattern

config: use --value instead of value-pattern

This option was introduced in a series of commits from fe3ccc7aab (Merge
branch 'ps/config-subcommands', 2024-05-15) and deprecated
`value-pattern`.  But `value-pattern` is still used throughout the doc.

The deprecated modes have been quarantined in the “Deprecated Modes”
section.  So let’s only use `--value=<pattern>` in the rest of the doc.

Acked-by: Patrick Steinhardt <redacted>
Signed-off-by: Kristoffer Haugsbakk <redacted>
Signed-off-by: Junio C Hamano <redacted>