Update vuln table

Author: suryansh-tob

not-so-smart-contracts/solana/README.md

@@ -15,6 +15,7 @@ Each _Not So Smart Contract_ includes a standard set of information:
 ## Vulnerabilities
 | Not So Smart Contract | Description |
 | --- | --- |
+| [Arbitrary CPI](arbitrary_cpi) | Arbitrary program account passed in upon invocation |
 | [Improper PDA Validation](improper_pda_validation) | PDAs are vulnerable to being spoofed via bump seeds |
 | [Ownership Check](ownership_check) | Broken access control due to missing ownership validation |
 | [Signer Check](signer_check) | Broken access control due to missing signer validation |