[SPARK-17010][MINOR][DOC] Wrong description in memory management document

## What changes were proposed in this pull request?

change the remain percent to right one.

## How was this patch tested?

Manual review

Author: Tao Wang <[email protected]>

Closes apache#14591 from WangTaoTheTonic/patch-1.

(cherry picked from commit 7a6a3c3)
Signed-off-by: Reynold Xin <[email protected]>

Author: WangTaoTheTonic

docs/tuning.md

@@ -115,7 +115,7 @@ Although there are two relevant configurations, the typical user should not need
 as the default values are applicable to most workloads:
 
 * `spark.memory.fraction` expresses the size of `M` as a fraction of the (JVM heap space - 300MB)
-(default 0.6). The rest of the space (25%) is reserved for user data structures, internal
+(default 0.6). The rest of the space (40%) is reserved for user data structures, internal
 metadata in Spark, and safeguarding against OOM errors in the case of sparse and unusually
 large records.
 * `spark.memory.storageFraction` expresses the size of `R` as a fraction of `M` (default 0.5).