Fix typos in README.md, CONTRIBUTORS.md

[zachkirsch]

Typo in the README.md

Looking at the source code (and the example in the Readme), constant-folding still occurs if the string length is 20.

Changed

Constant folding only occurs for strings having length less than 20.

To

Constant folding only occurs for strings having length less than or equal to 20

Typo in CONTRIBUTORS.md

Links were https:/github... (missing a slash)

[satwikkansal]

Thanks @zachkirsch :)