chore(internal): grammar fix (it's -> its)

Author: stainless-app[bot]

README.md

@@ -365,7 +365,7 @@ custom := param.Override[anthropic.FooParams](12)
 
 ### Request unions
 
-Unions are represented as a struct with fields prefixed by "Of" for each of it's variants,
+Unions are represented as a struct with fields prefixed by "Of" for each of its variants,
 only one field can be non-zero. The non-zero field will be serialized.
 
 Sub-properties of the union can be accessed via methods on the union struct.

packages/respjson/respjson.go

@@ -5,7 +5,7 @@ package respjson
 // Use [Field.Valid] to check if an optional value was null or omitted.
 //
 // A Field will always occur in the following structure, where it
-// mirrors the original field in it's parent struct:
+// mirrors the original field in its parent struct:
 //
 //	type ExampleObject struct {
 //		Foo bool	`json:"foo"`