Fix Diff documentation (#237)

The description inaccurately describes the operation of Diff,
which is y - x, where a '+' prefix denotes elements added from y
and a '-' prefix denotes elements removed from x.

For example:
	// Consider this call to Diff and its result.
	                  x  y
	cmp.Diff({b:2, c:3}, {a:1, b:2}) => {+a:1, b:2, -c:3}

	// Consider the same in mathematical notation.
	         y - x
	{a:1, b:2} - {b:2, c:3} = {+a:1, b:2, -c:3}

cmp/compare.go

@@ -100,8 +100,8 @@ func Equal(x, y interface{}, opts ...Option) bool {
 // same input values and options.
 //
 // The output is displayed as a literal in pseudo-Go syntax.
-// At the start of each line, a "-" prefix indicates an element removed from y,
-// a "+" prefix to indicates an element added to y, and the lack of a prefix
+// At the start of each line, a "-" prefix indicates an element removed from x,
+// a "+" prefix to indicates an element added from y, and the lack of a prefix
 // indicates an element common to both x and y. If possible, the output
 // uses fmt.Stringer.String or error.Error methods to produce more humanly
 // readable outputs. In such cases, the string is prefixed with either an