Fix grammar errors

"it's" is short for "it is", which is an easy test where it's "it's"
or "its" :-)

Author: moritz

doc/Language/typesystem.pod6

@@ -37,7 +37,7 @@ type constraint may change in this case.
 =head2 Type Objects
 
 To test if an object is a type object, test for definedness and check for
-identity between the object and it's C<.WHAT> pseudo-method. Note that the
+identity between the object and its C<.WHAT> pseudo-method. Note that the
 method C<.defined> can be overloaded and may provide false information.
 
     my $a = Int;
@@ -366,7 +366,7 @@ Defined as:
     multi sub trait_mod:<is>(Mu:U $child, Mu:U $parent)
 
 The L<trait|/type/Sub#Traits> X<C<is>|is (inheritance)> accepts a type object to be
-added as a parent class of a class in it's definition. To allow multiple
+added as a parent class of a class in its definition. To allow multiple
 inheritance the trait can be applied more then once. Adding parents to a class
 will import their methods into the target class. If the same method name occurs
 in multiple parents, the first added parent will win.
@@ -574,7 +574,7 @@ Default parameters can be provided.
 =head3 Roles as Types
 
 Roles can be used as type constraints wherever a type is expected. If a role is
-mixed in with C<does> or C<but>, it's type-object is added to the type-object
+mixed in with C<does> or C<but>, its type-object is added to the type-object
 list of the object in question. If a role is used instead of a class (using
 autopunning), the roles type-object is added to the inheritance chain.