"legder" docs typo

[Paul Morris]

Caught a couple of typos here:

http://www.lilypond.org/doc/v2.16/Documentation/notation/modifying-single-staves

Two corrections shown between [[[[ ... ]]]]

Legder [[[[ Ledger ]]]] lines can also be made to appear inside the staff where 
custom staff lines are required. The example shows the default position of 
ledger lines when the explicit legder-position [[[[ ledger-position ]]]] is and 
is not set.