syntax: c: increase the color contrast of reminders like FIXME

Bright white on yellow stands out much more than grey on yellow.
Signed-off-by: Liu Hao <lh_mouse@126.com>

syntax/c.nanorc

@@ -33,7 +33,7 @@ color brightblue "//.*"
 color brightblue start="/\*" end="\*/"
 
 # Reminders.
-color ,yellow "\<(FIXME|TODO|XXX)\>"
+color brightwhite,yellow "\<(FIXME|TODO|XXX)\>"
 
 # Trailing whitespace.
 color ,green "[[:space:]]+$"