Documentation Typo

[Vafilor]

Hi there,

I think there is a tiny typo in the documentation at 

https://www.gnu.org/software/make/manual/html_node/Special-Targets.html#Special-Targets under the .secondary name it says: 

Of course, of one of the source files were updated...

I believe it should be 

Of course, if one of the source files were updated...

so just changing the second `of` to be `if`. 

Thank you!

--

Andrey Melnikov