fix: Update cpi.mdx (solana-foundation#3694)

This commit fixes a comment
`AccountMeta::new_readonly(account4_pubkey, true)` should create the metadata for an account marked `non-writable` during an instruction

Author: elec7rik

docs/content/docs/basics/cpi.mdx

@@ -280,7 +280,7 @@ When building an instruction in Rust, use the following syntax to specify the
 AccountMeta::new(account1_pubkey, true),           // writable, signer
 AccountMeta::new(account2_pubkey, false),          // writable, not signer
 AccountMeta::new_readonly(account3_pubkey, false), // not writable, not signer
-AccountMeta::new_readonly(account4_pubkey, true),  // writable, signer
+AccountMeta::new_readonly(account4_pubkey, true),  // not writable, signer
 ```
 
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