Update README.md

Just spent several hours looking into a bug that really came down to me not presorting the array passed to quantile. Wasn't until the 5th time I went back to read the documentation that I caught that it was a sorted array, so I thought I'd extend the emphasis on array to sorted array.

Author: dyst5422

README.md

@@ -91,7 +91,7 @@ Returns the median of the given *array* of numbers using the [R-7 method](https:
 
 <a name="quantile" href="#quantile">#</a> d3.<b>quantile</b>(<i>array</i>, <i>p</i>[, <i>accessor</i>]) [<>](https://github.com/d3/d3-array/blob/master/src/quantile.js "Source")
 
-Returns the *p*-quantile of the given sorted *array* of numbers, where *p* is a number in the range [0, 1]. For example, the median can be computed using *p* = 0.5, the first quartile at *p* = 0.25, and the third quartile at *p* = 0.75. This particular implementation uses the [R-7 method](http://en.wikipedia.org/wiki/Quantile#Quantiles_of_a_population), which is the default for the R programming language and Excel. For example:
+Returns the *p*-quantile of the given *sorted array* of numbers, where *p* is a number in the range [0, 1]. For example, the median can be computed using *p* = 0.5, the first quartile at *p* = 0.25, and the third quartile at *p* = 0.75. This particular implementation uses the [R-7 method](http://en.wikipedia.org/wiki/Quantile#Quantiles_of_a_population), which is the default for the R programming language and Excel. For example:
 
 ```js
 var a = [0, 10, 30];