[gdb/doc] Fix gdb.unwinder docs

When building gdb with an older makeinfo (4.13), I run into:
...
gdb/doc/python.texi:3015: warning: `(' follows defined name \
  `gdb.unwinder.Unwinder.__init__' instead of whitespace.
gdb/doc/python.texi:3041: warning: `(' follows defined name \
  `gdb.unwinder.FrameId.__init__' instead of whitespace.
...

The warnings are related to these two lines:
...
@defun gdb.unwinder.Unwinder.__init__(name)
  ...
@defun gdb.unwinder.FrameId.__init__(sp, pc, special = @code{None})
...

Indeed, when checking the command and variable index, we can see that it
contains an incorrect entry:
...
  gdb.unwinder.FrameId.__init__(sp,:          Unwinding Frames in Python
...

Fix this by adding a space before the left parenthesis.

Tested by rebuilding the documentation and checking the command and variable
index.