Merge pull request #1550 from 0xPolygonZero/example-doc-fix

example documentation fix

plonky2/examples/range_check.rs

@@ -16,6 +16,8 @@ fn main() -> Result<()> {
 
     // The secret value.
     let value = builder.add_virtual_target();
+
+    // Registered as a public input (even though it's secret) so we can print out the value later.
     builder.register_public_input(value);
 
     let log_max = 6;