DOC: Fix stretch parameter typo in docstrings

[arsenelupin14]

Fixes a typo in the stretch parameter description in mpl_normalize.py and fits2bitmap.py where log' was written instead of 'log'. This unmatched quote confused the Sphinx parser, causing it to auto-link 'power' to the power unit.

Prompted by #19868

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[pllim]

While you did catch a missing quote, alas your patch does not fix the original reported problem. Please see the rendered doc from your PR: https://astropy--19871.org.readthedocs.build/en/19871/api/astropy.visualization.mpl_normalize.simple_norm.html#astropy.visualization.mpl_normalize.simple_norm

[pllim]

Hmm alas, my hunch was wrong. Your PR doc preview shows that this does not fix the ref. :(

You might have to look at https://github.com/astropy/sphinx-astropy/blob/66e6ba9b13c0869b2259a0726eaedf264def3553/sphinx_astropy/conf/v2.py#L135 or something over there...

[astrofrog]

@arsenelupin14 @pllim - I think astropy/sphinx-astropy#103 might be the proper fix for this (though we should still also do 980ea73 independently of this).

[pllim]

Let's just get the missing quotes in for now. Thanks!

[pllim]

I'll squash merge when I see RTD green.