Fix ruby code bug: (#27)

```
$ ruby -e "puts 'ruby'.starts_with?('r')"
-e:1:in `<main>': undefined method `starts_with?' for "ruby":String (NoMethodError)

puts 'ruby'.starts_with?('r')
           ^^^^^^^^^^^^^
Did you mean?  start_with?
$ ruby -e "puts 'ruby'.start_with?('r')"
true
```

Added in last PR by mistake.

build.rb

@@ -72,7 +72,7 @@ def fetch(url, name = nil)
 end
 
 # https://bugs.ruby-lang.org/issues/18658
-if stack == "heroku-22" && version.starts_with?("3.0")
+if stack == "heroku-22" && version.start_with?("3.0")
   raise "Cannot build Ruby 3.0 on heroku-22"
 end