chore: update Discovery.cpp (#473)

_[One line description of your change]_

### Motivation:

containined -> contained

### Modifications:

_[Describe the modifications you've done.]_

### Result:

_[After your change, what will change.]_

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: Ikko Eltociear Ashimine

Sources/_TestingInternals/Discovery.cpp

@@ -237,7 +237,7 @@ static SWTMachHeaderList getMachHeaders(void) {
       auto mhn = reinterpret_cast<SWTMachHeaderList::value_type>(mh);
 
       // Ignore this Mach header if it is in the shared cache. On platforms that
-      // support it (Darwin), most system images are containined in this range.
+      // support it (Darwin), most system images are contained in this range.
       // System images can be expected not to contain test declarations, so we
       // don't need to walk them.
       if (mhn->flags & MH_DYLIB_IN_CACHE) {