Added missing import

This was just a tad confusing for me. Might help others as well.

Author: MadsBogeskov

Sources/SwiftSyntax/Documentation.docc/Working with SwiftSyntax.md

@@ -188,6 +188,7 @@ trees when the `SwiftSyntaxBuilder` module is also imported:
 ```
 import SwiftSyntax
 import SwiftParser
+import SwiftSyntaxBuilder
 
 // `file` is a source file syntax node containing the parsed value of the
 // provided text.