Fix commas

Co-authored-by: P. L. Lim <2090236+pllim@users.noreply.github.com>

doc/customizing_guide/units.rst

@@ -7,8 +7,8 @@ Unit conversion in glue
 Data components can be assigned units as a string (or `None` to indicate no known units).
 By default, glue uses `astropy.units <https://docs.astropy.org/en/stable/units/index.html>`_ package
 to carry out unit conversions based on these units. However, it is possible to customize the
-unit conversion machinery, either to use a different unit transformation machinery, or to specify
-e.g. equivalencies in the astropy unit conversion. To customize the unit conversion behavior, you
+unit conversion machinery, either to use a different unit transformation machinery, or to specify,
+e.g., equivalencies in the astropy unit conversion. To customize the unit conversion behavior, you
 will need to define a unit converter as shown below::
 
     from astropy import units as u