fix one word for consistency.

R=gri
OCL=27543
CL=27543

doc/go_spec.html

@@ -3475,7 +3475,7 @@ must be assignment compatible to the iteration variables.
 <p>
 For strings, the "range" clause iterates over the Unicode code points
 in the string.  On successive iterations, the index variable will be the
-position of successive UTF-8-encoded code points in the string, and
+index of successive UTF-8-encoded code points in the string, and
 the second variable, of type <code>int</code>, will be the value of
 the corresponding code point.  If the iteration encounters an invalid
 UTF-8 sequence, the second variable will be <code>0xFFFD</code>,