Fix spelling mistakes in comments in shell scripts

gdb/ChangeLog:
2016-11-22  Ambrogino Modigliani  <ambrogino.modigliani@gmail.com>

        * contrib/expect-read1.sh: Fix spelling in comments.
        * gdb_buildall.sh: Fix spelling in comments.
        * gdb_mbuild.sh: Fix spelling in comments.
4 files changed