[MINOR] Modify the description of POST /sessions/{sessionId}/completion

[marblejenka]

What changes were proposed in this pull request?

Just modified a description of POST /sessions/{sessionId}/completion in the api-doc.

How was this patch tested?

Since the change is quite small, I didn't test the patch. If I have an instruction, I will follow it.

[codecov-io]

Codecov Report

Merging #285 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #285      +/-   ##
============================================
+ Coverage     68.24%   68.26%   +0.01%     
+ Complexity      965      964       -1     
============================================
  Files           104      104              
  Lines          5952     5952              
  Branches        900      900              
============================================
+ Hits           4062     4063       +1     
+ Misses         1311     1309       -2     
- Partials        579      580       +1     

Impacted Files	Coverage Δ	Complexity Δ
...ain/java/org/apache/livy/rsc/driver/RSCDriver.java	80.00% <0.00%> (-0.84%)	45.00% <0.00%> (-1.00%)
...main/scala/org/apache/livy/server/LivyServer.scala	33.03% <0.00%> (-0.45%)	11.00% <0.00%> (ø%)
...ain/scala/org/apache/livy/utils/SparkYarnApp.scala	75.00% <0.00%> (+1.25%)	40.00% <0.00%> (ø%)
...c/main/scala/org/apache/livy/repl/ReplDriver.scala	33.33% <0.00%> (+2.56%)	7.00% <0.00%> (ø%)
...cala/org/apache/livy/scalaapi/ScalaJobHandle.scala	55.88% <0.00%> (+2.94%)	7.00% <0.00%> (ø%)

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[jerryshao]

LGTM, merging to master branch.