rustc-dev-guide/index.md: add a missing period

[DanielEScherzer]

The other steps listed for subtree syncs end with a period

Rendered

[rustbot]

r? @ehuss

rustbot has assigned @ehuss.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Fallback group: @Mark-Simulacrum, internal-sites
• @Mark-Simulacrum, internal-sites expanded to Mark-Simulacrum, Urgau, ehuss, jieyouxu
• Random selection from Mark-Simulacrum, Urgau, ehuss, jieyouxu

[ehuss]

Thanks!