README.md: add missing comma

At least, I think it is a missing comma.
Could also have been a missing "that".

Of course, the typos are a sign that it was written by a human.
Weird world.
diff --git a/README.md b/README.md
index 374c8d2..1343936 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,7 @@
 # Sketch
 
 Sketch is an agentic coding tool focused on the Go programming language.
-Sketch runs in your terminal, has a web UI  understands your code, and helps you get work done.
+Sketch runs in your terminal, has a web UI, understands your code, and helps you get work done.
 To keep your environment pristine, sketch starts a docker container and outputs
 its work onto a branch in your host git repository.