Fix a typo (#391)

Closes #390

source/documentation/at-rules/use.html.md.erb

@@ -148,7 +148,7 @@ By default, a module's namespace is just the last component of its URL without a
 file extension. However, sometimes you might want to choose a different
 namespace—you might want to use a shorter name for a module you refer to a lot,
 or you might be loading multiple modules with the same filename. You can do this
-by writine `@use "<url>" as <namespace>`.
+by writing `@use "<url>" as <namespace>`.
 
 <% example do %>
   // src/_corners.scss