unsafe: fix typo in documentation of valid Pointer->uintptr->Pointer conversions

Change-Id: Ib669d5241372326a46361ee096570e960b7a957f
Reviewed-on: https://go-review.googlesource.com/19082Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/unsafe/unsafe.go

@@ -63,7 +63,7 @@ type ArbitraryType int
 // (3) Conversion of a Pointer to a uintptr and back, with arithmetic.
 //
 // If p points into an allocated object, it can be advanced through the object
-// by conversion to uintptr, addition of an offset, and conversion back to uintptr.
+// by conversion to uintptr, addition of an offset, and conversion back to Pointer.
 //
 //	p = unsafe.Pointer(uintptr(p) + offset)
 //