multiflags: fix doc warning properly

Rather than a dubious fix for a dubious warning, namely adding a
period after a parenthesized @Xref because the warning demands it, use
@pxref that is meant for exactly this case.  Thanks to Joseph Myers
for introducing me to it.


for  gcc/ChangeLog

	* doc/invoke.texi (multiflags): Drop extraneous period, use
	@pxref instead.

gcc/doc/invoke.texi

@@ -20588,8 +20588,8 @@ allocation before or after interprocedural optimization.
 @item -fmultiflags
 This option enables multilib-aware @code{TFLAGS} to be used to build
 target libraries with options different from those the compiler is
-configured to use by default, through the use of specs (@xref{Spec
-Files}.) set up by compiler internals, by the target, or by builders at
+configured to use by default, through the use of specs (@pxref{Spec
+Files}) set up by compiler internals, by the target, or by builders at
 configure time.
 
 Like @code{TFLAGS}, this allows the target libraries to be built for