Make English more idiomatic in README.
README: Correct typo, BDS -> BSD
Fixed some typos and grammar.

Some typos where found by aspell: `aspell -l en_US check README.md`
Fixed variable naming in README for sdscatrepr.
Fixed minor spelling typos in README.
Correct 'hold' typo in readme

A 'hold' is missing its 'h' and a variable was misnamed. This patch fixes these issues.

[antirez/sds#12 fixed the variable typos]