Fix typo in README for init.sql file location

[bumbummen99]

Corrected the default location for init.sql in the description above the example.

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Note

Low Risk
Documentation-only correction with no runtime or configuration behavior changes.

Overview
Fixes a documentation typo in the initialize with SQL file section: the default filesystem path for init.sql is now documented as /init.sql instead of /init.sh, matching the INIT_SQL override and the example volume mount in the same section.

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