[gdb/doc] Fix negative repeat count examining memory example

[gdb/doc] Fix negative repeat count examining memory example

The documentation for the examining memory command x contains an example:
...
You can also specify a negative repeat count to examine memory backward from
the given address.  For example, 'x/-3uh 0x54320' prints three halfwords (h)
at 0x54314, 0x54328, and 0x5431c.
...

The 0x54328 looks like a typo, which was intended to be 0x54318.

But the series uses a 4-byte distance, while the halfword size used in the
command means a 2-byte distance, so the series should be:
...
0x5431a, 0x5431c, and 0x5431e.
...

Fix this by updating the addresses in the example accordingly.

Reported here ( https://sourceware.org/pipermail/gdb/2021-November/049784.html
).