fix typo (#412)

Author: tshepang

src/how-to-build-and-run.md

@@ -160,7 +160,7 @@ The following tables indicate the outputs of various stage actions:
 | `stage0` builds `rustc` with `stage0-sysroot`             | `build/HOST/stage0-rustc/HOST`               |
 | copy `stage0-rustc (except executable)`                   | `build/HOST/stage0-sysroot/lib/rustlib/HOST` |
 | build `llvm`                                              | `build/HOST/llvm`                            |
-| `stage0` builds `codegen` with `stage0-sysroot`           | `build/HOST/stage0-codgen/HOST`              |
+| `stage0` builds `codegen` with `stage0-sysroot`           | `build/HOST/stage0-codegen/HOST`             |
 | `stage0` builds `rustdoc` with `stage0-sysroot`           | `build/HOST/stage0-tools/HOST`               |
 
 `--stage=0` stops here.