docs: remove quotes around undefined in Deno.env.get example (#29159)

`Deno.env.get` returns undefined when the environment variable is not
present. I initially interpreted this doc string to indicate that it
would return the string literal `"undefined"`, which isn't right.

Could use backticks here as above, but a little confusing in a JS
context where this also indicates a (template) string literal.

Author: SyntaxBlitz

cli/tsc/dts/lib.deno.ns.d.ts

@@ -1510,7 +1510,7 @@ declare namespace Deno {
      *
      * ```ts
      * console.log(Deno.env.get("HOME"));  // e.g. outputs "/home/alice"
-     * console.log(Deno.env.get("MADE_UP_VAR"));  // outputs "undefined"
+     * console.log(Deno.env.get("MADE_UP_VAR"));  // outputs undefined
      * ```
      *
      * Requires `allow-env` permission.