Fix typo in reference tables
The possessive form of "its" does not contain an apostrophe.
@@ -57,7 +57,7 @@ These are provided by the `.system()` method call.
| windows | Any version of Windows |
| cygwin | The Cygwin environment for Windows |
| haiku | |
-| freebsd | FreeBSD and it's derivatives |
+| freebsd | FreeBSD and its derivatives |
| dragonfly | DragonFly BSD |
| netbsd | |
