Sets correct syntax

As found by @taboege. If it's an bug in Rakudo, we still have to
reflect in the documentation whatever works. Closes #2878

Author: JJ

doc/Language/nativecall.pod6

@@ -658,11 +658,11 @@ equivalent signature as L<a constraint on the code parameter|/type/Signature#Con
 
     use NativeCall;
     # void SetCallback(int (*callback)(const char *))
-    my sub SetCallback(&callback:(Str --> int32)) is native('mylib') { * }
+    my sub SetCallback(&callback (Str --> int32)) is native('mylib') { * }
 
 Note: the native code is responsible for memory management of values passed to
-Perl 6 callbacks this way. In other words, NativeCall will not free() strings passed
-to callbacks.
+Perl 6 callbacks this way. In other words, NativeCall will not free() strings
+passed to callbacks.
 
 =head1 Library paths and names