Fixed typo in developer documentation (rust-lang#628)

Author: zhassan-aws

rmc-docs/src/dev-documentation.md

@@ -13,7 +13,7 @@
 ([`--keep-stage` comes with caveats](https://rustc-dev-guide.rust-lang.org/building/suggested.html#incremental-builds-with---keep-stage). Know that it may cause spurious build failures.)
 ```bash
 # Build rmc crate
-./scripts/setup/rmc_build_lib.sh
+./scripts/setup/build_rmc_lib.sh
 ```
 ```bash
 # Full regression suite