tour: fix grammatical error on Named Return Values page

Change-Id: Ide88384feaa257eab2df67912699f4c28240c859
Reviewed-on: https://go-review.googlesource.com/8667
Reviewed-by: Francesc Campoy Flores <campoy@golang.org>

content/basics.article

@@ -88,7 +88,7 @@ These names should be used to document the meaning of the return values.
 
 A `return` statement without arguments returns the current values of the results. This is known as a "naked" return.
 
-Naked return statements should be used only in short function, as with the example shown here. They can harm readability in longer functions.
+Naked return statements should be used only in short functions, as with the example shown here. They can harm readability in longer functions.
 
 .play basics/named-results.go