Update Libring coding rules

guidelines/Libring-coding-rules.md

@@ -38,9 +38,9 @@ developer members).
 
 ### Language Standard
 
-We SHALL use the **C++11 standard** syntax only.
+We SHALL use the **C++17 standard** syntax only.
 
-For gnu-gcc and clang, use compiler command line option `-std=c++11` to
+For gnu-gcc and clang, use compiler command line option `-std=c++17` to
 enforce this rule.
 
 ### Maximum Line Length