Corrected Ruby nutshell typo

Author: Mercerenies

doc/Language/rb-nutshell.pod6

@@ -289,7 +289,10 @@ add(2, 3);                       # => 5, and it works without it
 =end code
 
 =for code :lang<ruby>
-foo_method = &foo;     # Ruby
+def foo
+  ...
+end
+foo_method = method(:foo);     # Ruby
 =for code
 sub foo { ... };
 my &foo_method = &foo; # Raku