[3.13] gh-138295: Fix a grammar issue in the descriptor HOWTO (GH-138296) (GH-138300)

gh-138295: Fix a grammar issue in the descriptor HOWTO (GH-138296)

"an str" -> "a str"
(cherry picked from commit 552cf86b83e8aafebcfe9abbc77101bdbfda14c8)

Co-authored-by: Fangyi Zhou <me@fangyi.io>