Fix Japanese release note header to use 'リリースノート' (#35)

# Fix Japanese release note header

## Description
This PR fixes the Japanese release note header in
`jijmodeling-1.12.2.ipynb` to use "リリースノート" (in Japanese katakana)
instead of "Release Notes" (in English).

## Changes
- Updated the header in `docs/ja/releases/jijmodeling-1.12.2.ipynb` from
"JijModeling 1.12.2 Release Notes" to "JijModeling 1.12.2 リリースノート"

## Related PR
This change follows the corresponding update in JijModelingAlpha:
Jij-Inc/JijModelingAlpha#498

## Link to Devin run
https://app.devin.ai/sessions/37da6a793565435599477c81bd2649a0

## Requested by
h.ishii@j-ij.com

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: h.ishii@j-ij.com <h.ishii@j-ij.com>

Author: devin-ai-integration[bot]

docs/ja/releases/jijmodeling-1.12.2.ipynb

@@ -8,7 +8,7 @@
     }
    },
    "source": [
-    "# JijModeling 1.12.2 Release Notes\n",
+    "# JijModeling 1.12.2 リリースノート\n",
     "\n",
     "## 機能強化"
    ]