fix duplicated word

R=rsc
DELTA=1  (0 added, 0 deleted, 1 changed)
OCL=33788
CL=33791

doc/go_spec.html

@@ -2412,7 +2412,7 @@ second is a value of the channel's element type.
 <p>
 Except for shift operations,
 if one operand has ideal type and the other operand does not,
-the ideal operand is converted converted to match the type of
+the ideal operand is converted to match the type of
 the other operand (§<a href="#Expressions">Expressions</a>).
 If both operands are ideal numbers and one is an
 ideal float, the other is converted to ideal float