Fix correlation query example in README (#610)

Updated span_id variable in correlation query example.

Signed-off-by: Zhu Jiekun <jiekun@victoriametrics.com>

Author: jiekun

src/README.md

@@ -221,7 +221,7 @@ feature for navigating from a span in a trace directly to logs relevant for that
 
 An example of the correlation query in traces datasource is the following:
 ```
-trace_id:="${__trace.traceId}" AND span_id:="${__trace.spanId}"
+trace_id:="${__trace.traceId}" AND span_id:="${__span.spanId}"
 ```
 
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