.travis.yml: fix typo Ubutnu -> Ubuntu

Author: kdudka

.travis.yml

@@ -9,7 +9,7 @@ matrix:
         - name: "Ubuntu LTS 2016 (xenial)"
           dist: xenial
 
-        - name: "Ubutnu LTS 2018 (bionic)"
+        - name: "Ubuntu LTS 2018 (bionic)"
           dist: bionic
 
         - name: "Ubuntu LTS 2020 (focal)"