Fix RemoteChannel example in parallel-computing.md

The example demonstrating the use of RemoteChannel does not work in Julia 0.7+,
due to new scoping of variables inside of for loops. Consequently, the example
would not work as intended. Adding global scope to the variable corrects this
problem.

(cherry picked from commit 082ec88)

doc/src/manual/parallel-computing.md

@@ -1088,7 +1088,7 @@ julia> for p in workers() # start tasks on the workers to process requests in pa
 julia> @elapsed while n > 0 # print out results
            job_id, exec_time, where = take!(results)
            println("$job_id finished in $(round(exec_time; digits=2)) seconds on worker $where")
-           n = n - 1
+           global n = n - 1
        end
 1 finished in 0.18 seconds on worker 4
 2 finished in 0.26 seconds on worker 5