Update _base.less

Fixed typos

library/less/_base.less

@@ -16,7 +16,7 @@ and will be used across all viewports.
 03. LAYOUT & GRID STYLES
 04. LINK STYLES
 05. H1, H2, H3, H4, H5 STYLES
-06. HEADER SYTLES
+06. HEADER STYLES
 07. NAVIGATION STYLES
 08. POSTS & CONTENT STYLES
 09. PAGE NAVI STYLES
@@ -170,7 +170,7 @@ h5, .h5 {
 }
 
 /*********************
-06. HEADER SYTLES
+06. HEADER STYLES
 *********************/
 
 .header {