Fix span start typo (#5365)

Author: martin308

content/en/docs/languages/swift/instrumentation.md

@@ -77,7 +77,7 @@ operation. Spans are the building blocks of Traces. To create a span use the
 span builder associated with the tracer:
 
 ```swift
-let span =  let builder = tracer.spanBuilder(spanName: "\(name)").startSpan()
+let span = tracer.spanBuilder(spanName: "\(name)").startSpan()
 ...
 span.end()
 ```