Fix doc in "2D Navigation Overview"

[urtow]

navigation_agent have method called get_next_path_location(), not get_next_location()

[mhilbrunner]

cc @smix8

[paulloz]

If the method was renamed, it also needs to be changed in the C# example (line 182).

[urtow]

@paulloz updated.

P.S. Nice avatar

[smix8]

Should also fix the setter function not just the getter.

[akien-mga]

Thanks for the contribution! This was missing some places that need to be changed, and actually using the wrong name for the new method (get_next_path_position, not _location). So I made #6910 to fix it.