Fix SPARK_ACCESS_KEY comment

In the makefile ```core-framework/build/makefile``` there is a comment just before the ```program-cloud:``` recipe:
```
# Program the core using the cloud. SPARK_CORE_ID and SPARK_ACCESS_KEY must
# have been defined in the environment before invoking 'make program-cloud'
program-cloud: $(TARGET).bin
```
As it turns out the text should say SPARK_ACCESS_TOKEN the clue being:
```
# URL to invoke cloud flashing
CLOUD_FLASH_URL = https://api.spark.io/v1/devices/$(SPARK_CORE_ID)\?access_token=$(SPARK_ACCESS_TOKEN)
```