python: fix Linetable case to LineTable in docstrings and comments

The class is called LineTable, not Linetable, as specified by
py-linetable.c/gdbpy_initialize_linetable:

    if (gdb_pymodule_addobject (gdb_module, "LineTable",

gdb/ChangeLog:

        * python/py-linetable.c: Fix case of Linetable to LineTable
        in docstrings and code comments.
        * python/py-symtab.c: Same.