Fixed a broken link in the OTel module.

The content has moved to another repo open-telemetry/semantic-conventions.
To make the link more future proof, it now points to the official site.

Fixes #149.

Author: pluknet

xml/en/docs/ngx_otel_module.xml

@@ -281,7 +281,9 @@ The value can contain variables.
 <section id="span" name="Default span attributes">
 
 <para>
-The following <link url="https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/semantic_conventions/http.md">span attributes</link>
+The following
+<link url="https://opentelemetry.io/docs/specs/semconv/registry/attributes/http/">span
+attributes</link>
 are added automatically:
 
 <list type="bullet">