Fix typo in fcn.md (#412)
Fix typo in fcn.md (#412)

S/ResNet 18/ResNet-18/g
d2l-ai/d2l-zh,mli/gluon-tutorials-zh,mli/gluon-tutorials-zh,d2l-ai/d2l-zh,d2l-ai/d2l-zh,mli/gluon-tutorials-zh,mli/gluon-tutorials-zh