function-arg-return: fix docs typo (#504)

Signed-off-by: Stephan Renatus <stephan@styra.com>

docs/rules/style/function-arg-return.md

@@ -32,7 +32,7 @@ has_email(user) if {
 ## Rationale
 
 Older Rego policies sometimes contain an unusual way to declare where the return value of a function call should be
-stored — the last argument of the function. True to it's Datalog roots, return values may be stored either using
+stored — the last argument of the function. True to its Datalog roots, return values may be stored either using
 assignment (i.e. `:=`) or by appending a variable name to the argument list of a function. While both forms are valid,
 using assignment `:=` consistently is preferred.