docs: Fix 'modff' reference in extend.texi

docs: Fix 'modff' reference in extend.texi

In commit a2a919aa501e3 (2003), built-ins for modf and modff were added.
In extend.texi, section "Other Builtins", "modf" was added to the paragraph
"There are also built-in versions of the ISO C99 functions [...]" and
"modf" was also added to the paragraph "The ISO C90 functions [...]".
"modff" was not added to either paragraph.

Based on the context clues about where "modfl" and other similar function
pairs like "powf/powl" appear, I believe the reference to "modf" in the
first paragraph (C99) should instead be "modff".

2022-04-25  Paul A. Clarke  <pc@us.ibm.com>

gcc
	* doc/extend.texi (Other Builtins): Correct reference to 'modff'.