syimage -> sysimage in "Upgrading from..." section (#628)

Author: chriselrod

docs/src/index.md

@@ -81,8 +81,8 @@ PackageCompiler 2.0 comes with a few breaking changes.
   create an alias or shortcut that starts Julia with a custom sysimage by specifying the `--sysimage=<PATH/TO/SYSIMAGE>` 
   command line option.
 - Lazy artifacts (those not downloaded until used) are not included in apps by default anymore. Use `include_lazy_artifacts=true` to re-enable this.
-- Passing no packages to `create_sysimage` will now include all packages in the given project instead of a syimage with no packages.
+- Passing no packages to `create_sysimage` will now include all packages in the given project instead of a sysimage with no packages.
   Use `String[]` as a first argument if you want the old behavior.
 - The `audit_app` function has been removed. It caught too few problems to be of useful in practice.
 - The keyword `app_name` in `create_app` has been removed and replaced with a more flexible version.
-  If you used `app_name="Foo"`, replace it with `executables=["Foo"=>"julia_main"]`.
+  If you used `app_name="Foo"`, replace it with `executables=["Foo"=>"julia_main"]`.