Fix incorrect sample code in Trace/README.md (#1604)
Fix incorrect sample code in Trace/README.md (#1604)

the sample code in Trace/README.md is incorrectly.
it should be 
`setStartTime` and `setEndTime`
also check:
https://github.com/googleapis/google-cloud-php/blob/master/Trace/src/Span.php#L215
https://github.com/googleapis/google-cloud-php/blob/master/Trace/src/Span.php#L253
jdpedrie/google-cloud-php,jdpedrie/google-cloud-php,chingor13/google-cloud-php,googleapis/google-cloud-php,googleapis/google-cloud-php,googleapis/google-cloud-php,chingor13/google-cloud-php,jdpedrie/google-cloud-php,chingor13/google-cloud-php