Small fix in backporting.md - remove unneeded trailing . for git remote add command

Summary: fixes typo in backporting.md

Test Plan: Try the original command, note failure on git fetch. Try without ., note success on git fetch.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4633