ENGCOM-1502: [Backport] Fix typo in less button definition #15108

 - Merge Pull Request #15108 from sergiy-v/magento2:feature/2.1-buttons-less-typo
 - Merged commits:
   1. 12bf629

Author: magento-engcom-team

lib/web/css/source/lib/_buttons.less

@@ -8,7 +8,7 @@
 //  _____________________________________________
 
 //
-//  Button with solid or pictire background
+//  Button with solid or picture background
 //  ---------------------------------------------
 
 .lib-button(