[AST] fix typo in PluginRegistry.h

exectuable -> executable

Author: Ikko Eltociear Ashimine

include/swift/AST/PluginRegistry.h

@@ -40,7 +40,7 @@ class LoadedLibraryPlugin {
   void *getAddressOfSymbol(const char *symbolName);
 };
 
-/// Represent a "resolved" exectuable plugin.
+/// Represent a "resolved" executable plugin.
 ///
 /// Plugin clients usually deal with this object to communicate with the actual
 /// plugin implementation.