Documentation: doc-guide: fixes to sphinx.rst

Documentation: doc-guide: fixes to sphinx.rst

Various fixes to sphinx.rst:

- eliminate a double-space between 2 words
- grammar/wording
- punctuation
- call rows in a table 'rows' instead of 'columns' (or does Sphinx
  call everything a column?)
- It seems that "amdfonts" should be "amsfonts". I can't find any
  amdfonts.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/20201228231212.22448-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>