Plagiarism is when you take someone's work and use it as your own. If you were to copy and essay written by someone else and use that as your own work, then yes.
But based on what you are asking, if you are taking your own essay, written by you, and posting your essay online for corrections, then no. Because that essay you want corrections on is still your own, and you are the rightful owner.
I hope this helps! Let me know if you don't understand