Merge pull request #440 from carolabadeer/fix-php-custom-attributes-code-snippet

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

Author: vasireddy99

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"]);