Update README release build instruction

The instructions did not use `as prod` for the `tar` `rebar3 tar` task, which
would create the tarball in `_build/default`, but the `install.sh` expects to
find the tar file in `_build/prod`. This updates the readme to instruct the
user how to create the tar in the correct location for the install script.

Signed-off-by: Winford <winford@object.stream>

README.md

@@ -1,4 +1,4 @@
-# `atomvvm_packbeam`
+# `atomvm_packbeam`
 
 An Erlang Escript and library used to generate an [AtomVM](http://github.com/atomvm/AtomVM) AVM file from a set of files (beam files, previously built AVM files, or even arbitrary data files).
 
@@ -19,7 +19,7 @@ Building `packbeam` requires a version of Erlang/OTP compatible with [AtomVM](ht
 To build a release, run the following commands:
 
     shell$ rebar3 release
-    shell$ rebar3 tar
+    shell$ rebar3 as prod tar
 
 These commands will create an Erlang tar archive containing a versioned release of the `atomvm_packbeam` tool, e.g.,