Fix description of spark.speculation.quantile

## What changes were proposed in this pull request?

Minor doc fix regarding the spark.speculation.quantile configuration parameter.  It incorrectly states it should be a percentage, when it should be a fraction.

## How was this patch tested?

(Please explain how this patch was tested. E.g. unit tests, integration tests, manual tests)

(If this patch involves UI changes, please attach a screenshot; otherwise, remove this)

I tried building the documentation but got some unidoc errors.  I also got them when building off origin/master, so I don't think I caused that problem.  I did run the web app and saw the changes reflected as expected.

Author: Nicholas Brown <nbrown@adroitdigital.com>

Closes #14352 from nwbvt/master.

docs/configuration.md

@@ -1174,7 +1174,7 @@ Apart from these, the following properties are also available, and may be useful
   <td><code>spark.speculation.quantile</code></td>
   <td>0.75</td>
   <td>
-    Percentage of tasks which must be complete before speculation is enabled for a particular stage.
+    Fraction of tasks which must be complete before speculation is enabled for a particular stage.
   </td>
 </tr>
 <tr>