[gdb/testsuite] Update INTERNAL_GDBFLAGS example

In commit 31c50280 ("[gdb/testsuite] Add -q to INTERNAL_GDBFLAGS") I added
-q to the INTERNAL_GDBFLAGS, but I forgot to update the INTERNAL_GDBFLAGS
example in gdb/testsuite/README.

Fix this by adding the -q there as well.