Fix Rack::Lint error message for HTTP_CONTENT_TYPE and HTTP_CONTENT_L…

…ENGTH (#2007)

Currently it's printing:

```
Rack::Lint::LintError: env contains HTTP_CONTENT_TYPE, must use
```

Which had me puzzled for quite a while.

Co-authored-by: Jean Boussier <jean.boussier@gmail.com>

lib/rack/lint.rb

@@ -303,7 +303,7 @@ def check_environment(env)
         ## (use the versions without <tt>HTTP_</tt>).
         %w[HTTP_CONTENT_TYPE HTTP_CONTENT_LENGTH].each { |header|
           if env.include? header
-            raise LintError, "env contains #{header}, must use #{header[5, -1]}"
+            raise LintError, "env contains #{header}, must use #{header[5..-1]}"
           end
         }