spec: clarify sentence about non-constant shifts

Fixes #10514.

Change-Id: Iae95a304d3ebb1ed82567aa234e05dc434db984f
Reviewed-on: https://go-review.googlesource.com/13098Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

doc/go_spec.html

@@ -3320,7 +3320,7 @@ to the type of the other operand.
 The right operand in a shift expression must have unsigned integer type
 or be an untyped constant that can be converted to unsigned integer type.
 If the left operand of a non-constant shift expression is an untyped constant,
-the type of the constant is what it would be if the shift expression were
+it is first converted to the type it would assume if the shift expression were
 replaced by its left operand alone.
 </p>