[docs] Fix stale sink.writer-coordinator.cache-memory default value

[thswlsqls]

Purpose

• The "Write Initialize" section states the default of sink.writer-coordinator.cache-memory as 1GB, but the source default is 2GB.
• FlinkConnectorOptions.SINK_WRITER_COORDINATOR_CACHE_MEMORY is defined as defaultValue(MemorySize.ofMebiBytes(2048)) (= 2GB).
• Commit abb779813 raised the default from ofMebiBytes(1024) to ofMebiBytes(2048), but this hand-written prose was never updated. Fixes the doc-vs-code drift so users tune Job Manager memory with the correct value.

Tests

• Documentation-only wording fix, no behavior change — no test added.
• Corrected value verified against FlinkConnectorOptions.java (ofMebiBytes(2048) = 2GB). Docs-only change, no module build.

[JingsongLi]

Can you just merge all doc related PRs to a single?

[thswlsqls]

@JingsongLi Makes sense — thanks for the guidance. I'll batch documentation fixes into a single PR going forward instead of opening one per change.