Fix some typos and grammar mistakes found using LanguageTool

[Calinou]

There are still a lot of mistakes to fix, this is just an initial pass.

Command used (has many false positives, beware):

parallel java -jar languagetool-commandline.jar --language en-US --languagemodel ~/Documents/ngrams -d ARROWS,COMMA_PARENTHESIS_WHITESPACE,DASH_RULE,DOUBLE_PUNCTUATION,EN_QUOTES,ENGLISH_WORD_REPEAT_BEGINNING_RULE,ENGLISH_WORD_REPEAT_RULE,MORFOLOGIK_RULE_EN_US,MULTIPLICATION_SIGN,PUNCTUATION_PARAGRAPH_END,PLUS_MINUS,SENTENCE_WHITESPACE,TR,UNLIKELY_OPENING_PUNCTUATION,UPPERCASE_SENTENCE_START,WHETHER,WHITESPACE_RULE,WORD_CONTAINS_UNDERSCORE {} ::: "$PWD"/^classes/**/*.rst

[clayjohn]

Woah! Lots of stuff here. Its surprising how many errors we miss on review.

[akien-mga]

Thanks!