[docs] updated info on the C++ standard used in Kudu code

The Apache Kudu project has switched to using the C++-17 standard a long
time ago, so it's time to update the corresponding paragraph in the
documentation.

Change-Id: I51112c8e9fbc189da7b48526414af28bdcf29af9
Reviewed-on: http://gerrit.cloudera.org:8080/20859
Tested-by: Kudu Jenkins
Reviewed-by: Abhishek Chennaka <achennaka@cloudera.com>
1 file changed