Minor typo in R-exts

Josh O'Brien 
    on Mon, 22 Jul 2019 15:09:09 -0700 writes:

> Section 1.1.3 of the "R-exts" manual currently refers to "imports or
    > importFrom directives" where it means to say "import or importFrom
    > directives". Results of a diff between R-exts.texi at SVN revision
    > 76864 and a corrected version are copied below.

Thank you, Josh.   Fixed in the sources now.

Martin