[gdb/doc] Use '@:' after abbreviation

In gdb/doc/gdb.texinfo, we have:
...
For every type there is also a default kind associated with it, e.g.@
@code{Integer} in @value{GDBN} will internally be an @code{Integer*4} (see the
table below for default types).
...

While the @NL command [1] does probably prevent the sentence ending at 'e.g.',
the usual way to accomplish this is to use '@:' [2], so use that instead.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=33868

Approved-By: Eli Zaretskii <eliz@gnu.org>

[1] https://www.gnu.org/software/texinfo/manual/texinfo/html_node/Multiple-Spaces.html
[2] https://www.gnu.org/software/texinfo/manual/texinfo/html_node/Not-Ending-a-Sentence.html