Remove extra space after @pxref in gdb.texinfo

Internally at AdaCore, documentation is still built with Texinfo 4.13.
This version gave an error when building gdb.texinfo:

../../../binutils-gdb/gdb/doc/gdb.texinfo:27672: @pxref expected braces.
../../../binutils-gdb/gdb/doc/gdb.texinfo:27672: ` {dotdebug_gdb_scripts section,,The @cod...' is too long for expansion; not expanded.

... followed by many more spurious errors that were caused by this
one.

This patch fix the problem by removing the extra space.

I don't know whether it's advisable to try to support this ancient
version of Texinfo (released in 2008 apparently); but in this
particular case the fix is trivial, so I'm checking it in.

gdb/doc/ChangeLog
2021-01-27  Tom Tromey  <tromey@adacore.com>

	* gdb.texinfo (Auto-loading extensions): Remove extraneous space.