fix(docs): Fix documentation for default value of rollingPercentilesE…

…nabled (#563)

rollingPercentilesEnabled option is documented as defaulting to false,
but rolling percentiles are enabled by default.

lib/circuit.js

@@ -52,7 +52,7 @@ Please use options.errorThresholdPercentage`;
  * @param {boolean} options.rollingPercentilesEnabled This property indicates
  * whether execution latencies should be tracked and calculated as percentiles.
  * If they are disabled, all summary statistics (mean, percentiles) are
- * returned as -1. Default: false
+ * returned as -1. Default: true
  * @param {Number} options.capacity the number of concurrent requests allowed.
  * If the number currently executing function calls is equal to
  * options.capacity, further calls to `fire()` are rejected until at least one