Windows CI: spell it MSYS2

Author: MisterDA

.github/workflows/windows.yml

@@ -38,7 +38,7 @@ jobs:
             D:\opamroot
           key: ${{ runner.os }}-${{ matrix.build_env }}-opam-${{ hashFiles('install.ps1') }}
 
-      - name: Add MSys2 to PATH and install prerequisites
+      - name: Add MSYS2 to PATH and install prerequisites
         if: matrix.build_env == 'msys2'
         run: |
           "C:\msys64" | Out-File -FilePath $env:GITHUB_PATH -Encoding utf8 -Append