page-menu.css: better inset shadow

before (glitchy shadow at the top):  
![](https://d2oawfjgoy88bd.cloudfront.net/5461e80038d387141ab4b739/5461e9c638d387141c52bfb7/549050a300779135586ff073.png?Expires=1418830388&Signature=A9e~cdXc6OdPD1Fo2PsH2Cv92vKncM~u0Bfg4~T~Weu2VMxO7eiNtK5cQbW4ZPdpPqT7gcRxIfRTJtDoq0N8v3fxlpGHuP8pkDVaN~g3B8FEx3Y-3gw7UuW-p4vz6EhytEwk8waVs0UIgnDu48bBOfIIFIt3XC0BaoGxvgt-iXlr~IlEDBR2xx0m-UGchR5KYtxnUStveX0txBuzVNsqBJnH3pzP8~Hnl5-fR8f0N6eI-jDHsWOy~GQoW5Qth7F0RiN4u~pobZf4pOJNSGRfCx8ADutOEPqYOOqnUZuNbfNutwu~L-uNRqGEc1D0JteuHRjNXjOTJpYG0C2W-cHfVw__&Key-Pair-Id=APKAJHEJJBIZWFB73RSA)

after:  
![](https://d2oawfjgoy88bd.cloudfront.net/5461e80038d387141ab4b739/5461e9c638d387141c52bfb7/549050c600779135586ff079.png?Expires=1418830410&Signature=OjZk3ZAUhYoCr8qzuOGnBjt7RlpGPWCbG3t06TAs5VM7mFRyUMlkAhMoq0y~9DL5kdjzpKVwkN7m-qupgl7lFg6aKdaNjaXfDVD4rfk4aSxnV5DdrrumZPp8~1WCKCF65BDcrbpCktHXnti~0Ln~aT7EGzGOzmcXzkcjONJmD-j5pu505oJQgEzDZhSPgpoJnQF9EG2TDrN4AmS4SkKEnkEsJDC6HBKXL7tbb-eoSYu03FAoXzL04d6fmvDSttfRBSVno7sADcpMEv4yYYOteM~lZcDbgMnbQcDJud7DlSr1s5zyKECOnv7vQiyEacvnn5vbDU9SfLGikBGAfldErA__&Key-Pair-Id=APKAJHEJJBIZWFB73RSA)

courtesy of @brumm