Fix typos around parallel dependency execution in task.md (#1294)

Co-authored-by: Jo Franchetti <jofranchetti@gmail.com>

Author: dandv

runtime/reference/cli/task.md

@@ -109,9 +109,9 @@ Task serve deno run -RN server.ts
 Listening on http://localhost:8000/
 ```
 
-Dependency tasks are in parallel, with the default parallel limit being equal to
-number of cores on your machine. To change this limit use `DENO_JOBS`
-environmental variable.
+Dependency tasks are executed in parallel, with the default parallel limit being
+equal to number of cores on your machine. To change this limit, use the
+`DENO_JOBS` environmental variable.
 
 Dependencies are tracked and if multiple tasks depend on the same task, that
 task will only be run once: