Docs: made suggested fix

Author: Sl1mb0

src/parallel-rustc.md

@@ -13,7 +13,7 @@ The underlying thread-safe data-structures used in the parallel compiler
 can be found in `rustc_data_structures/sync.rs`. Some of these data structures
 use the `parking_lot` API.
 
-## Code Gen
+## Codegen
 
 During [monomorphization][monomorphization] the compiler splits up all the code to 
 be generated into smaller chunks called _codegen units_. These are then generated by