Fix incorrect precision example in GeometricWidthDiscretiser docs

[maupatel]

The GeometricWidthDiscretiser guidance on the precision parameter gives an inconsistent worked example.

precision is the number of decimal places (forwarded to pd.cut(..., precision=...)), so precision = N corresponds to a smallest step of 10**-N. The tip establishes this with its first example (precision = 3 -> 0.001), but then states precision = 7 -> 0.0001, which is 10**-4, not 10**-7. The correct value is 0.0000001.

Fixed in both places the sentence appears (kept in sync):

• feature_engine/discretisation/geometric_width.py (docstring)
• docs/user_guide/discretisation/GeometricWidthDiscretiser.rst

Documentation-only; no code or behavior changes.