gdb/doc: use 'standard error stream' instead of 'stderr' in some places

gdb/doc: use 'standard error stream' instead of 'stderr' in some places

With this commit:

  commit 91f2597bd24d171c1337a4629f8237aa47c59082
  Date:   Thu Aug 12 18:24:59 2021 +0100

      gdb: print backtrace for internal error/warning

I included some references to 'stderr', which, it was pointed out,
would be better written as 'standard error stream'.  See:

  https://sourceware.org/pipermail/gdb-patches/2021-September/182225.html

This commit replaces the two instances of 'stderr' that I introduced.