docs(design): address barth's comments

Co-authored-by: John Molakvoæ <skjnldsv@users.noreply.github.com>
Signed-off-by: Nimisha Vijay <52440189+nimishavijay@users.noreply.github.com>

Author: nimishavijay

developer_manual/design/foundations.rst

@@ -149,7 +149,7 @@ Fonts
 Font sizes
 ^^^^^^^^^^
 
--  ``--defalut-font-size``\ (15px): For body text
+-  ``--default-font-size``\ (15px): For body text
 -  ``--font-size-small`` (13px): For nonessential information
 -  Other than those 2 font sizes, 20px is also often used for headings, and 24px for titles.