[PATCH] Cygwin: doc: Fix type mismatch in porting.texi

Issue:newlib/libgloss/doc/porting.texi:570: warning: @ref node name should not contain `.'
      newlib/libgloss/doc/porting.texi:747: warning: @ref node name should not contain `.'
      newlib/libgloss/doc/porting.texi:938: warning: @ref node name should not contain `.'

reproduce:
git clone https://github.com/riscv-collab/riscv-gnu-toolchain.git
cd riscv-gnu-toolchain
./configure --prefix=`pwd`/install --with-arch=rv64gc --with-abi=lp64d --enable-multilib
make -j $(nproc)

The above issue will happen when executing the build command. This patch eliminates these warnings.
@ref{That},'That' does not contain node;
I think it's just appropriate to use @file here,
@file{file-name},'file-name' support node;
refer:https://www.gnu.org/software/texinfo/manual/texinfo/texinfo.html#g_t_0040file
---
 libgloss/doc/porting.texi | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libgloss/doc/porting.texi b/libgloss/doc/porting.texi
index 15f214d02..4a269a970 100644
--- a/libgloss/doc/porting.texi
+++ b/libgloss/doc/porting.texi
@@ -567,7 +567,7 @@ and then the ROM takes over. Pick a safe vector with no side
 effects. Some ROMs have a builtin trap handler just for this case.
 @end enumerate
 portable between all the m68k based boards we have here.
-@ref{crt0.S,,Example Crt0.S}. 
+@file{crt0.S,,Example Crt0.S}.
 
 
 @smallexample
@@ -744,7 +744,7 @@ check a few critical addresses like @code{start}, @code{bss_end}, and
 
 Here's a breakdown of a linker script for a m68k based target board.
 See the file @code{libgloss/m68k/idp.ld}, or go to the appendixes in
-the end of the manual. @ref{idp.ld,,Example Linker Script}. 
+the end of the manual. @file{idp.ld,,Example Linker Script}.
 
 @smallexample
 STARTUP(crt0.o)
@@ -935,7 +935,7 @@ sbrk(). @code{malloc()}, @code{calloc()}, and @code{realloc()} all call
 @code{sbrk()} at there lowest level. @code{caddr_t} is defined elsewhere
 as @code{char *}. @code{RAMSIZE} is presently a compile time option. All
 this does is move a pointer to heap memory and check for the upper
-limit. @ref{glue.c,,Example libc support code}. @code{sbrk()} returns a
+limit. @file{glue.c,,Example libc support code}. @code{sbrk()} returns a
 pointer to the previous value before more memory was allocated.
 
 @smallexample
-- 
2.17.1