Werk #11131: filestats: fix typo and grammar

Component	Setup
Title	filestats: fix typo and grammar
Date	Jun 29, 2020
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.0.0i1 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT

Fix some typos and grammar issues in the description text in the WATO plugin

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