DEV: stop freezing frozen strings

We have the `# frozen_string_literal: true` comment on all our
files. This means all string literals are frozen. There is no need
to call #freeze on any literals.

For files with `# frozen_string_literal: true`

```
puts %w{a b}[0].frozen?
=> true

puts "hi".frozen?
=> true

puts "a #{1} b".frozen?
=> true

puts ("a " + "b").frozen?
=> false

puts (-("a " + "b")).frozen?
=> true
```

For more details see: https://samsaffron.com/archive/2018/02/16/reducing-string-duplication-in-ruby

lib/plain_text_to_markdown.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class PlainTextToMarkdown
-  SIGNATURE_SEPARATOR ||= "-- ".freeze
+  SIGNATURE_SEPARATOR ||= "-- "
 
   def initialize(plaintext, opts = {})
     @plaintext = plaintext