[fs.path.native.obs,fs.path.generic.obs] Qualify std::format (cplusplus#8350)

Fixes NB US 190-305 (C++26 CD).

Author: jwakely

source/iostreams.tex

@@ -14727,7 +14727,7 @@
 \begin{itemdescr}
 \pnum
 \returns
-\tcode{format("\{\}", *this)}.
+\tcode{std::format("\{\}", *this)}.
 \begin{note}
 The returned string is suitable for use with formatting\iref{format.functions}
 and print functions\iref{print.fun}.
@@ -14801,7 +14801,7 @@
 \begin{itemdescr}
 \pnum
 \returns
-\tcode{format("\{:g\}", *this)}.
+\tcode{std::format("\{:g\}", *this)}.
 \begin{note}
 The returned string is suitable for use with formatting\iref{format.functions}
 and print functions\iref{print.fun}.