Fix grammar in Adaptive Scheduler documentation

Correct grammatical error in the description of the Adaptive Scheduler.

Author: Maestro2496

docs/content/docs/deployment/elastic_scaling.md

@@ -31,7 +31,7 @@ This page describes a new class of schedulers that allow Flink to adjust job's p
 
 ## Adaptive Scheduler
 
-The Adaptive Scheduler can adjust the parallelism of a job based on available slots. It will automatically reduce the parallelism if not enough slots are available to run the job with the originally configured parallelism; be it due to not enough resources being available at the time of submission, or TaskManager outages during the job execution. If new slots become available the job will be scaled up again, up to the configured parallelism.
+The Adaptive Scheduler can adjust the parallelism of a job based on available slots. It will automatically reduce the parallelism if not enough slots are available to run the job with the originally configured parallelism; be it is due to not enough resources being available at the time of submission, or TaskManager outages during the job execution. If new slots become available the job will be scaled up again, up to the configured parallelism.
 
 In Reactive Mode (see below) the configured parallelism is ignored and treated as if it was set to infinity, letting the job always use as many resources as possible.