Fixed wrong font include.

This fixes https://dev.vaadin.com/ticket/19800

Change-Id: I9b12c76b97af5731a4dd374ecab5b217c404a04d

documentation/themes/themes-fonts.asciidoc

@@ -19,7 +19,7 @@ follows:
 
 ----
 @include font(MyFontFamily,
-              '../../mytheme/fonts/myfontfamily');
+              '../../../../mytheme/fonts/myfontfamily');
 ----
 
 The statement must be given in the [filename]#styles.scss# file __outside__ the