This issue is happening because of you are using both the latest bootstrap beta version (v4.0.0-beta.3) and earlier bootstrap version (v3.3.7). I believe you have used both the following cdn path.
https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.3/css/bootstrap.min.css
https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css
If you use the above one, you will end up with the result as you showed in the image. Same you can verify in the FIDDLE DEMO.
The solution is to use the version(v3.3.7) only in your code and you can get the same look and feel as they have showed in bootstrap site. Same you can check in this FIDDLE DEMO
NOTE: Whenever you post some question in StackOverflow, add minimal running code of your problem, so that people will easily understand your problem and help you to resolved the issue. Only IMAGE won’t work for all the time.
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solved Bootstrap navbar issues