Clarify configuring sampling in the collector

https://www.jaegertracing.io/docs/sampling/

(a) change "Collector Sampling Configuration" section title to "Collector Configuration", looks better in the TOC in the right sidebar

(b) explain mounting config file for Docker. From gitter

```
maniaabdi @maniaabdi 16:19
Hello everyone, how should we specify sampling strategies to jaeger-collector. I am using the following command
sudo docker run --rm -p14268:14268 -p9411:9411 -p14267:14267 -e CASSANDRA_SERVERS=192.168.100.15 -e CASSANDRA_PORT=9042 jaegertracing/jaeger-collector --sampling.strategies-file=/home/ubuntu/strategies.json
and I keep getting
{"level":"fatal","ts":1534364309.442946,"caller":"collector/main.go:228","msg":"Failed to create sampling strategy store","error":"Failed to open strategies file: open /home/ubuntu/strategies.json: no such file or directory","errorVerbose":"open /home/ubuntu/strategies.json: no such file or directory\nFailed to open strategies
but I am sure that my file exist
any help is really appreciated

Won Jun Jang @black-adder 16:26
https://stackoverflow.com/questions/41092587/passing-file-as-argument-to-docker-container
you have to pass the file to the container as a volume

Prithvi Raj @vprithvi 16:26
(because the file isn't in the container)

Won Jun Jang @black-adder 16:26
-v /home/ubuntu/strategies.json:/home/ubuntu/strategies.json should work
```

Open the sampling documentation page linked in the issue and locate the “Collector Sampling Configuration” section. Rename that section to “Collector Configuration” and add an explanation of mounting the sampling strategies file into the Docker container, using the reported volume-mount scenario as context. Done means the rendered page has the updated title and Docker guidance.

Written by the indexing model from the issue text.