Fix doubled word typos.

Doc/api/newtypes.tex

@@ -22,7 +22,7 @@ defining new object types.
   Initialize a newly-allocated object \var{op} with its type and
   initial reference.  Returns the initialized object.  If \var{type}
   indicates that the object participates in the cyclic garbage
-  detector, it it added to the detector's set of observed objects.
+  detector, it is added to the detector's set of observed objects.
   Other fields of the object are not affected.
 \end{cfuncdesc}