docs: fix simple typo, architecure -> architecture

There is a small typo in builtin/README.md.

Should read `architecture` rather than `architecure`.

builtin/README.md

@@ -142,7 +142,7 @@ different compilers or architectures.
 
 Creating implementations of one compiler's builtins using builtins
 from another is probably your best bet to improve performance.
-Another useful possibility is using architecure-specific builtins, or
+Another useful possibility is using architecture-specific builtins, or
 even embedded assembly, when they are available.
 
 ### Architecture-specific builtins