go/spec: fix typo

R=r
CC=golang-dev
https://golang.org/cl/1790041

doc/go_spec.html

@@ -995,7 +995,7 @@ must either all be present or all be absent. If present, each name
 stands for one item (parameter or result) of the specified type; if absent, each
 type stands for one item of that type.  Parameter and result
 lists are always parenthesized except that if there is exactly
-one unnamed result it may written as an unparenthesized type.
+one unnamed result it may be written as an unparenthesized type.
 </p>
 <p>
 If the function's last parameter has a type prefixed with <code>...</code>,