Fix misleading comment

`gsub(/\/\z/, '')` removes a leading slash (`/`) not a space

```
> 'https://gitlab.com/'.gsub(/\/\z/, '')
=> "https://gitlab.com"

> 'https://gitlab.com/  '.gsub(/\/\z/, '')
=> "https://gitlab.com/  "
```

lib/sentry/client.rb

@@ -206,7 +206,7 @@ module Sentry
 
       uri = URI(url)
       uri.path.squeeze!('/')
-      # Remove trailing spaces
+      # Remove trailing slash
       uri = uri.to_s.gsub(/\/\z/, '')
 
       uri