[fs.class.path] Use "typedef-name" instead of "typedef" (cplusplus#8964)

Author: TymianekPL

source/iostreams.tex

@@ -13789,7 +13789,7 @@
 
 \indexlibrarymember{value_type}{path}%
 \pnum
-\tcode{value_type} is a \keyword{typedef} for the
+\tcode{value_type} is a \grammarterm{typedef-name} for the
 operating system dependent encoded character type used to represent pathnames.
 
 \indexlibrarymember{preferred_separator}{path}%