docs: fix 'it's' to 'its' typo in TypeScript guide

Fix possessive pronoun typo in the useContext typing section.
'it's' (contraction of 'it is') should be 'its' (possessive).

Co-Authored-By: Ian Alloway <adapter_burners.1y@icloud.com>

Author: devin-ai-integration[bot]

src/content/learn/typescript.md

@@ -286,7 +286,7 @@ export default App = AppTSX;
 
 This technique works when you have a default value which makes sense - but there are occasionally cases when you do not, and in those cases `null` can feel reasonable as a default value. However, to allow the type-system to understand your code, you need to explicitly set `ContextShape | null` on the `createContext`.
 
-This causes the issue that you need to eliminate the `| null` in the type for context consumers. Our recommendation is to have the Hook do a runtime check for it's existence and throw an error when not present:
+This causes the issue that you need to eliminate the `| null` in the type for context consumers. Our recommendation is to have the Hook do a runtime check for its existence and throw an error when not present:
 
 ```js {5, 16-20}
 import { createContext, useContext, useState, useMemo } from 'react';