[entgrpc] fix to get singular name

[lesomnus]

.EntEdge.StructField has string value with pascal case so "Children" is stored which cannot be singulerized by the sigular function that only takes lower case.

.EntEdge.Name has same string with .EntEdge.StructField but lower case so "children" is stored; now singular returns "child". Also it needs to be transformed into pascalcase again.

ref. https://github.com/ent/ent/blob/092b62a3229476d18d9ffade87da268d9136f13a/entc/gen/template/builder/update.tmpl#L207