README Ignition source-build example points to stale v0.45.1 tag

Problem

The README's network-version table says Fuel Ignition, Testnet, and Devnet currently use Fuel Core 0.48.1, but the source-build instructions for running an Ignition node still tell users to check out v0.45.1:

git checkout v0.45.1

That command sits under the sentence "Go to the latest release tag for ignition", so the two README sections contradict each other.

Expected behavior

The source-build example should point at the currently documented Ignition network tag, or explicitly explain why v0.45.1 is intentionally required.

Suggested fix

Update the checkout example to v0.48.1 so it matches the "Versions currently used in networks" table.