fix grammatical typo in docs (#6034)

very simple fix in the [broadcasting by dimension name](http://xarray.pydata.org/en/stable/user-guide/computation.html#broadcasting-by-dimension-name) docs:

> ``DataArray`` objects ~~are~~ automatically align themselves [...] by dimension name instead of axis order.

Author: delgadom

doc/user-guide/computation.rst

@@ -546,7 +546,7 @@ two gaussian peaks:
 Broadcasting by dimension name
 ==============================
 
-``DataArray`` objects are automatically align themselves ("broadcasting" in
+``DataArray`` objects automatically align themselves ("broadcasting" in
 the numpy parlance) by dimension name instead of axis order. With xarray, you
 do not need to transpose arrays or insert dimensions of length 1 to get array
 operations to work, as commonly done in numpy with :py:func:`numpy.reshape` or