dbm.whichdb docstring shows outdated return-value example

dbm.whichdb() returns the dotted submodule name, such as "dbm.ndbm" or
"dbm.gnu" (these are the keys of the internal _modules dispatch table, and
the values documented in Doc/library/dbm.rst).

The function docstring in Lib/dbm/__init__.py still shows the pre-package
form:

- the name of the dbm submodule (e.g. "ndbm" or "gnu") if recognized.

The example should use the actual dotted names, to match the code and the
documentation.

Linked PRs

• gh-153306

開啟 Lib/dbm/init.py 並找到 dbm.whichdb() 的 docstring。更新其回傳值範例，使其使用 issue 中描述的點號分隔子模組名稱，然後確認它是否與 Doc/library/dbm.rst 中記載的值相符。