[MINOR][DOC] improve the doc for "spark.memory.offHeap.size"

The description of "spark.memory.offHeap.size" in the current document does not clearly state that memory is counted with bytes....

This PR contains a small fix for this tiny issue

document fix

Author: CodingCat <zhunansjtu@gmail.com>

Closes #11561 from CodingCat/master.

docs/configuration.md

@@ -769,7 +769,7 @@ Apart from these, the following properties are also available, and may be useful
   <td><code>spark.memory.offHeap.size</code></td>
   <td>0</td>
   <td>
-    The absolute amount of memory which can be used for off-heap allocation.
+    The absolute amount of memory in bytes which can be used for off-heap allocation.
     This setting has no impact on heap memory usage, so if your executors' total memory consumption must fit within some hard limit then be sure to shrink your JVM heap size accordingly.
     This must be set to a positive value when <code>spark.memory.offHeap.enabled=true</code>.
   </td>