[SPARK-10695] [DOCUMENTATION] [MESOS] Fixing incorrect value informati…

…on for spark.mesos.constraints parameter.

Author: Akash Mishra <akash.mishra20@gmail.com>

Closes #8816 from SleepyThread/constraint-fix.

docs/running-on-mesos.md

@@ -189,10 +189,10 @@ using `conf.set("spark.cores.max", "10")` (for example).
 You may also make use of `spark.mesos.constraints` to set attribute based constraints on mesos resource offers. By default, all resource offers will be accepted.
 
 {% highlight scala %}
-conf.set("spark.mesos.constraints", "tachyon=true;us-east-1=false")
+conf.set("spark.mesos.constraints", "tachyon:true;us-east-1:false")
 {% endhighlight %}
 
-For example, Let's say `spark.mesos.constraints` is set to `tachyon=true;us-east-1=false`, then the resource offers will be checked to see if they meet both these constraints and only then will be accepted to start new executors.
+For example, Let's say `spark.mesos.constraints` is set to `tachyon:true;us-east-1:false`, then the resource offers will be checked to see if they meet both these constraints and only then will be accepted to start new executors.
 
 # Mesos Docker Support