Docs: Fix tag-replacement example in Java API quickstart

**Apache Iceberg version** main @ 49b89a8c5 **Query engine** None (Java API) **Please describe the bug** The "Replacing and fast forwarding branches and tags" example in `docs/docs/java-api-quickstart.md` (line 255) calls `table.manageSnapshots().replaceBranch(tag, 4)` on `tag = "audit-tag"`, a tag (already used as a tag two examples earlier via `useRef("audit-tag")`). `UpdateSnapshotReferencesOperation.replaceBranch(String, long)` (`core/src/main/java/org/apache/iceberg/UpdateSnapshotReferencesOperation.java` line 99-104) requires the target ref to be a branch (`Preconditions.checkArgument(ref.isBranch(), "Ref %s is a tag not a branch", name)`), so calling it on a tag throws `IllegalArgumentException` at runtime. `ManageSnapshots` already exposes `replaceTag(String, long)` (`api/src/main/java/org/apache/iceberg/ManageSnapshots.java` line 154) for exactly this case. **Steps to reproduce** Copy the quickstart example as written and run it against a table with an `audit-tag` tag: the call to `replaceBranch(tag, 4)` throws `IllegalArgumentException: Ref audit-tag is a tag not a branch`. **Additional context** N/A.