trace: fix NewLimitedSampler example in documentation.
trace: fix NewLimitedSampler example in documentation.

Fixes #436.

Change-Id: I2e7f453971a972133e422c4c860e9af4c49f6769
Reviewed-on: https://code-review.googlesource.com/9790
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go