Fix span variable name in PHP - Adding Custom Attributes code snippet

Author: carolabadeer

src/docs/getting-started/php-sdk/trace-manual-instr.mdx

@@ -220,7 +220,7 @@ $span = $tracer
         ->spanBuilder('span')
         ->startSpan();
 
-$spanScope = $root->activate();
+$spanScope = $span->activate();
 
 $span->setAttributes(["a" => "1"]);