spelling: extension

Signed-off-by: Josh Soref <jsoref@users.noreply.github.com>

Author: jsoref

lib/SymbolGraphGen/Symbol.cpp

@@ -324,7 +324,7 @@ static SubstitutionMap getSubMapForDecl(const ValueDecl *D, Type BaseType) {
     return {};
 
   // Map from the base type into the this declaration's innermost type context,
-  // or if we're dealing with an extention rather than a member, into its
+  // or if we're dealing with an extension rather than a member, into its
   // extended nominal (the extension's own requirements shouldn't be considered
   // in the substitution).
   swift::DeclContext *DC;