Bump stable version referenced in readme (#33984)

Author: cmcaine

README.md

@@ -84,7 +84,7 @@ Julia. However, most users should use the most recent stable version
 of Julia. You can get this version by changing to the Julia directory
 and running:
 
-    git checkout v1.2.0
+    git checkout v1.3.0
 
 Now run `make` to build the `julia` executable.