correct identation error

Author: carlosuc3m

bioimageio/core/build_spec/build_model.py

@@ -429,7 +429,7 @@ def addMissingAxes(im_axes):
             needed_axes = ["b", "c", "x", "y", "z", "s"]
             for ax in needed_axes:
                 if not ax in im_axes:
-                im_axes += ax
+                    im_axes += ax
             return im_axes
 
         axes_ij = "bzcyxs"