Update structures.rakudoc

Fixed grammar.

Author: ZzZombo

doc/Language/structures.rakudoc

@@ -291,7 +291,7 @@ say %hash-plus.last;    # OUTPUT: «4 => 44␤»
 In C<Lastable> we use the universal C<self> variable to refer to whatever object
 this particular role is mixed in; in this case it will contain the hash it is
 mixed in with; it will contain something else (and possibly work some other way)
-in other case. This role will provide the C<last> method to any variable it's
+in another case. This role will provide the C<last> method to any variable it's
 mixed with, providing new, attachable, functionalities to I<regular> variables.
 Roles can even be L<added to existing variables using the C<does> keyword|/language/objects#Mixins_of_roles>.