typo fix (MINOR) (#39)

Author: isopov

wrap.go

@@ -6,7 +6,7 @@ var (
 
 	// Private error type for non-errors errors, used as a not-nil substitute that cannot be type-checked directly
 	foreignType = syntheticErrors.NewType("foreign")
-	// Private error type used as a universal wrapper, meant to ann nothing at all to the error apart from some message
+	// Private error type used as a universal wrapper, meant to add nothing at all to the error apart from some message
 	transparentWrapper = syntheticErrors.NewType("decorate").ApplyModifiers(TypeModifierTransparent)
 	// Private error type used as a densely opaque wrapper which hides both the original error and its own type
 	opaqueWrapper = syntheticErrors.NewType("wrap")