Updating latest stable version number in README (#42543)

Changing the latest stable version number in the "Building Julia" section of the REAMDE from `v1.6.2` to `v1.6.3` (according to [the downloads page](https://julialang.org/downloads/#current_stable_release)).
JuliaLang · Oct 8, 2021 · 8f660fe · 8f660fe
1 parent 18771fc
commit 8f660fe
Showing 1 changed file with 1 addition and 1 deletion.
diff --git a/README.md b/README.md
@@ -93,7 +93,7 @@ Julia. However, most users should use the [most recent stable version](https://g
 of Julia. You can get this version by changing to the Julia directory
 and running:
 
-    git checkout v1.6.2
+    git checkout v1.6.3
 
 Now run `make` to build the `julia` executable.