Small fix to gdb.Value constructor doc

Small fix to gdb.Value constructor doc

The synopsis of the two-parameters form of the gdb.Value constructor is
currently shown as

    Value.__init__ (val, [, type ])

in the documentation.

First, there is an extra comma, which I think we can remove in any
case.

Then, since the type parameter is not optional, I would not put in
between square brackets.  Those usually indicate that something is
optional.

With this patch, it appears as:

    Value.__init__ (val, type)

gdb/doc/ChangeLog:

	* python.texi (Values From Inferior): Change synopsys of the
	second form of Value.__init__.