Fixed a typo in CONTRIBUTING.md
Small change to the CONTRIBUTING.md file.
r? @steveklabnik
This is the fastest way to recompile after you changed only rustc source code.
Note however that the resulting rustc binary won't have a stdlib to link
against by default. You can build libstd once with `x.py build src/libstd`,
- but it is is only guaranteed to work if recompiled, so if there are any issues
+ but it is only guaranteed to work if recompiled, so if there are any issues
recompile it.
- `x.py test` - build the full compiler & run all tests (takes a while). This
is what gets run by the continuous integration system against your pull