Fix typo in README.md for 'tskit' format (#3349)

Author: hyanwong

README.md

@@ -11,7 +11,7 @@
 [![Docs Build](https://github.com/tskit-dev/tskit/actions/workflows/docs.yml/badge.svg)](https://github.com/tskit-dev/tskit/actions/workflows/docs.yml) [![Binary wheels](https://github.com/tskit-dev/tskit/actions/workflows/wheels.yml/badge.svg)](https://github.com/tskit-dev/tskit/actions/workflows/wheels.yml) [![Tests](https://github.com/tskit-dev/tskit/actions/workflows/tests.yml/badge.svg)](https://github.com/tskit-dev/tskit/actions/workflows/tests.yml)
 
 
-The succinct tree sequence (`tskit) format is an efficient way of representing
+The succinct tree sequence (`tskit`) format is an efficient way of representing
 the genetic history - sometimes known as an Ancestral Recombination Graph or ARG -
 of a set of related DNA sequences. `Tskit` is used
 by a number of software libraries and programs (such as