docs: migrate config v1 to v2

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

docs/tracing.md

@@ -9,6 +9,8 @@ By configuring `io.containerd.tracing.processor.v1.otlp` plugin.
 containerd daemon can send traces to the specified OpenTelemetry endpoint.
 
 ```toml
+version = 2
+
 [plugins."io.containerd.tracing.processor.v1.otlp"]
     endpoint = "http://localhost:4318"
 ```
@@ -26,6 +28,8 @@ The sampling ratio and the service name on the traces could be configured by
 `io.containerd.internal.v1.tracing` plugin.
 
 ```toml
+version = 2
+
 [plugins."io.containerd.internal.v1.tracing"]
     sampling_ratio = 1.0
     service_name = "containerd"