CompatHelper: bump compat for Flux to 0.15 for package docs, (keep existing compat)

Author: CompatHelper Julia

docs/Project.toml

@@ -63,7 +63,7 @@ DifferentialEquations = "7"
 Distributions = "0.25"
 Documenter = "1"
 DomainSets = "0.6, 0.7"
-Flux = "0.13, 0.14"
+Flux = "0.13, 0.14, 0.15"
 ForwardDiff = "0.10"
 IncompleteLU = "0.2"
 Integrals = "4"