Remove use of "beta" from Readme

Remove's "beta" from description of R package in the readme. Technically we're haven't done the 1.0 release, but it's quite stable and several people have expressed confusion over whether we consider it unstable based on the use of "beta" to describe the R package releases.

Author: jgabry

README.md

@@ -34,7 +34,7 @@ analysis.
 
 #### Installing the R package
 
-You can install the latest beta release of the **cmdstanr** R package with
+You can install the latest release of the **cmdstanr** R package with
 
 ```r
 # we recommend running this in a fresh R session or restarting your current session