Fix README; stack.offset can’t be an array.

README.md

@@ -1080,9 +1080,9 @@ The stack order is computed prior to the [offset](#stack_offset); thus, the lowe
 
 <a name="stack_offset" href="#stack_offset">#</a> <i>stack</i>.<b>offset</b>([<i>offset</i>]) · [Source](https://github.com/d3/d3-shape/blob/master/src/stack.js)
 
-If *offset* is specified, sets the offset accessor to the specified function or array and returns this stack generator. If *offset* is not specified, returns the current offset acccesor, which defaults to [stackOffsetNone](#stackOffsetNone); this uses a zero baseline. See [stack offsets](#stack-offsets) for the built-in offsets.
+If *offset* is specified, sets the offset accessor to the specified function and returns this stack generator. If *offset* is not specified, returns the current offset acccesor, which defaults to [stackOffsetNone](#stackOffsetNone); this uses a zero baseline. See [stack offsets](#stack-offsets) for the built-in offsets.
 
-If *offset* is a function, it is passed the generated series array and the order index array. The offset function is then responsible for updating the lower and upper values in the series array to layout the stack. For example, the default offset is defined as:
+The offset function is passed the generated series array and the order index array; it is then responsible for updating the lower and upper values in the series array. For example, the default offset is defined as:
 
 ```js
 function offsetNone(series, order) {