Fix typo in suggested "rvm autolibs" command

In the upgrade notes text for RVM 1.20, it was suggested to run "rvm autolibs read fail" to revert to the pre-1.20 autolibs setting.  The actual setting is "read-fail", which has an equivalent integer value of 2.  If the command is executing as given, with a space instead of a dash, the setting will be effectively changed to "read-only" (equivalent integer value 1):

```
turnip:~ blandingj$ rvm autolibs read fail
turnip:~ blandingj$ rvm autolibs status
---
value: read
number: 1
runner: osx
description: Allow RVM to use package manager if found but do not install or fail if dependencies are missing.
```

This could cause RVM to ignore missing library requirements when installing rubies.