Extra dollar sign to avoid env var substitution in docs

[daddykotex]

Previous implementation left the dollar sign as is, this triggered a warning:

[warn] [bootstrapped] /Users/David.Francoeur/workspace/dev/smithy4s/modules/bootstrapped/src/generated/smithy4s/example/EnvVarString.scala:12:38: Variable ENV undefined in comment for class EnvVarString in class EnvVarString
[warn] [bootstrapped]   *   This is meant to be used with $ENV_VAR
[warn] [bootstrapped]                                      ^
[warn] [bootstrapped] one warning found

Current implementation uses a double $ sign to avoid this. I tried using {@literal $} but they must be using some sort of basic regex that fails, because when I try, it fails with an Illegal group reference similar to this SO question

[Baccata]

Good thinking !

[daddykotex]

I got curious and wondered why literal failed, and looking into the compile I found: https://github.com/scala/scala/blob/40beefbb750d326a5ae71ed852c58dc9a31d7e17/src/scaladoc/scala/tools/nsc/doc/base/CommentFactoryBase.scala#L143

which just wraps the character in backtick. So I gave it a shot and it works:

Do you mind if I open another PR for that?

[daddykotex]

Did it anyway: #1242

We can close if not needed