Fix HDF5 example

docs/src/examples/hdf5.md

@@ -14,7 +14,7 @@ using Random
 
 Random.seed!(23)  # to make things reproducible ;)
 
-f = H5File("foo.h5")
+f = H5File("foo.h5"; mode="w")
 ```
 
 We now have an `H5File` instance which we can use to store datasets.

src/hdf5/hdf5.jl

@@ -40,7 +40,7 @@ struct H5File
     _h5f::HDF5.File
     _datasets::Dict{String, H5CompoundDataset}
 
-    function H5File(fname::AbstractString, mode="r")
+    function H5File(fname::AbstractString; mode="r")
         h5f = h5open(fname, mode)
         if mode != "r"
             if "KM3io.jl" ∈ keys(attrs(h5f))