Minor fixes to the introduction and a normalization pass for abbreviations

[ayshih]

• Minor fixes to the introduction
• Normalization pass for abbreviations (see https://github.com/sunpy/sunpy-1.0-paper/pull/149#issuecomment-522408736). To be listed in the table, an abbreviation needs to be used at least twice in the text, and the Map/TimeSeries table is ignored in that counting, as well as the parenthetical following "HTTP websites" in the Fido section.