C++: Fix placeholder syntax in training example

Author: adityasharad

docs/language/ql-training/query-examples/cpp/data-flow-cpp-2.ql

@@ -2,11 +2,11 @@ import cpp
 import semmle.code.cpp.dataflow.DataFlow
 import semmle.code.cpp.commons.Printf
 
-class SourceNode extends DataFlow::Node { ... }
+class SourceNode extends DataFlow::Node { /* ... */ }
 
 from FormattingFunction f, Call c, SourceNode src, DataFlow::Node arg
 where c.getTarget() = f and
       arg.asExpr() = c.getArgument(f.getFormatParameterIndex()) and
       DataFlow::localFlow(src, arg) and
       not src.asExpr() instanceof StringLiteral
-select arg, "Non-constant format string."
+select arg, "Non-constant format string."