Program security - PDA sharing lesson updated (#449)

* Updated code snippets and content as per guidelines

* Fixed transfer_ctx and spl import

* Fixed transfer_ctx and spl import

Author: 0xCipherCoder

content/courses/onchain-development/anchor-pdas.md

@@ -5,7 +5,7 @@ objectives:
   - Enable and use the `init_if_needed` constraint
   - Use the `realloc` constraint to reallocate space on an existing account
   - Use the `close` constraint to close an existing account
-description: 
+description:
   "Store arbitrary data on Solana, using PDAs, an inbuilt key-value store."
 ---