docs: fix f-string prefix and typo in plugin-hooks example

[vanshtaneja23]

Two small fixes in docs/plugins/plugin-hooks.md, register_fragment_loaders section:

1. Line 244: remove stray double period (callback.. → callback.).
2. Lines 281-282: add missing f prefix on the second example's fragment source strings. Without it, {argument} renders literally instead of interpolating. The first example (line 269) does this correctly via .format(argument); this makes the second consistent.

Verified by running the interpolation locally: without the f, the source is the literal my-fragments:{argument}.