[3.14] gh-134449: fix grammar for precision_with_grouping in format description (GH-134608) (#135015)

gh-134449: fix grammar for `precision_with_grouping` in format description (GH-134608)

This amends commit f39a07b.
(cherry picked from commit 7828d52)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>

Author: miss-islington

Doc/library/string.rst

@@ -328,7 +328,7 @@ The general form of a *standard format specifier* is:
    sign: "+" | "-" | " "
    width_and_precision: [`width_with_grouping`][`precision_with_grouping`]
    width_with_grouping: [`width`][`grouping`]
-   precision_with_grouping: "." [`precision`][`grouping`]
+   precision_with_grouping: "." [`precision`][`grouping`] | "." `grouping`
    width: `~python-grammar:digit`+
    precision: `~python-grammar:digit`+
    grouping: "," | "_"