Fix typos and grammar in docs and issue templates

[olitreadwell]

What changed

Fixes five small, genuine typos and grammar errors across the docs and the issue templates.

• Filter Reference: "and then outputs the result" → "and then output the result" (subject-verb agreement).
• Docs guide: "a few hundreds pages" → "a few hundred pages".
• Bug and feature issue templates: "Verify this steps" → "Verify these steps".
• README: correct the test:coverage line to match the command's own description ("coverage report for core tests"), instead of "App tests".

Why

These are small but user-facing errors. The issue templates' instructions and the Filter Reference read as mistakes, and the README mis-describes what thor test:coverage actually does. Self-found gaps, no open issue.

How to verify

Docs-only change; no Ruby or asset code is touched, so the existing test suite is unaffected.

git fetch https://github.com/olitreadwell/devdocs.git chore/fix-typos-and-grammar
git checkout FETCH_HEAD

The diff is limited to five markdown files, each a single-word correction. Review each changed word against its surrounding sentence.