Punctuation fix.

Punctuation is important in texts. You need to pay attention to them.

I changes 'it's' to 'its', because it will mean another thing to use 'it's' instead of 'its'.

CONTRIBUTING.md

@@ -74,7 +74,7 @@ As of v3.2.0, Font Awesome's CSS, LESS, SCSS, and documentation are all powered
 - Multiple-line approach (one property and value per line)
 - Always a space after a property's colon (.e.g, `display: block;` and not `display:block;`)
 - End all lines with a semi-colon
-- For multiple, comma-separated selectors, place each selector on it's own line
+- For multiple, comma-separated selectors, place each selector on its own line
 - Attribute selectors, like `input[type="text"]` should always wrap the attribute's value in double quotes, for consistency and safety (see this [blog post on unquoted attribute values](http://mathiasbynens.be/notes/unquoted-attribute-values) that can lead to XSS attacks)