Fixing compile error in Anchor CPI lesson (#636)

* Fixing compile error in Anchor CPI lesson

* Fixed prettier error

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

@@ -362,6 +362,17 @@ anchor-lang = { version = "0.30.1", features = ["init-if-needed"] }
 anchor-spl = "0.30.1"
 ```
 
+Since we are adding `anchor-spl` as a dependency we also need to add the
+`idl-build` for it in the features section of `Cargo.toml`. This is because all
+types that will be used in the `Accounts` structures that we are adding in this
+lesson require the `IdlBuild` trait implementation to generate an IDL.
+
+```rust
+[features]
+# All lines remain unchanged, except for this idl-build line
+idl-build = ["anchor-lang/idl-build", "anchor-spl/idl-build"]
+```
+
 ### Initialize reward token
 
 Next, navigate to `lib.rs` and implement the `InitializeMint` context type and