Fixed a failing test of documentation build

In nitpicking mode, sphinx was producing these warnings "WARNING:
py:obj reference target not found:...". I am not sure what is the
origin of these warnings for the `all_world2pix` function but it may be
due to its interesting way of creating the docstring and "injecting" it
into the function. In any case, for each problematic object I have
changed single backquotes to double backquotes.