Fix docs notebook: per-fg unit on the child-count threshold

The 'tax deduction only for families with at least two children' example
compares `familie__anzahl_kinder_fg` (a count at the Familiengemeinschaft
level) against a threshold parameter. Under GEP 10 a comparison requires both
operands in the same unit, so the threshold must be declared
`PERSON_COUNT.PER_FG`, not plain `PERSON_COUNT`. Without this the docs build
fails with a UnitConsistencyError while executing the notebook.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: MImmesberger

docs/how_to_guides/modifications_of_policy_environments.ipynb

@@ -1010,8 +1010,11 @@
     "# Step 2: Create a new parameter `min_anzahl_kinder_für_kinderfreibetrag`\n",
     "from gettsim.tt import ScalarParam, TTSIMUnit\n",
     "\n",
+    "# The threshold is compared against `familie__anzahl_kinder_fg`, a child count at the\n",
+    "# Familiengemeinschaft level, so it carries the same per-fg unit (GEP 10): a comparison\n",
+    "# is only meaningful between operands in identical units.\n",
     "min_anzahl_kinder_für_kinderfreibetrag = ScalarParam(\n",
-    "    value=2, unit=TTSIMUnit.PERSON_COUNT\n",
+    "    value=2, unit=TTSIMUnit.PERSON_COUNT.PER_FG\n",
     ")\n",
     "\n",
     "# Step 3: Create a new `PolicyFunction` that modifies the tax deduction for children\n",