GDB/doc: Remove indentation from `print -elements' completion example

Remove indentation from the text of the manual after the example here:

"  Completion will in some cases guide you with a suggestion of what
kind of argument an option expects.  For example:

     (gdb) print -elements <TAB><TAB>
     NUMBER     unlimited

   Here, the option expects a number (e.g., '100'), not literal
'NUMBER'.  Such metasyntactical arguments are always presented in
uppercase."

as this is a continuation of the same paragraph.