Fix minor typo in AIP-0203 (#1353)

Renames it's -> its possessive in AIP-203

Author: martinbjeldbak

aip/general/0203.md

@@ -81,7 +81,7 @@ a message.
 ### Immutable
 
 The use of `IMMUTABLE` indicates that a field on a resource cannot be changed
-after it's creation. This can apply to either fields that are input or outputs,
+after its creation. This can apply to either fields that are input or outputs,
 required or optional.
 
 When a service receives an immutable field in an update request (or similar),