gdb/python: fix 'set python ignore-environment' white space

gdb/python: fix 'set python ignore-environment' white space

I noticed that the help text for set/show python ignore-environment
was messed up, some lines had unwanted leading white space, like this:

  (gdb) help set python ignore-environment
  Set whether the Python interpreter should ignore environment variables.
   When enabled GDB's Python interpreter will ignore any Python related
          flags in the environment.  This is equivalent to passing `-E' to a
          python executable.
  (gdb)

This has been present since the ignore-environment setting was added
in commit:

  commit edeaceda7b2f33b2c3bf78c732e67f3188e7f0b9
  Date:   Thu Aug 27 16:53:13 2020 +0100

      gdb: startup commands to control Python extension language

Fixed in this commit.