doc/muxers/segment: clarify ambiguos reference

Replace "The segmenter muxer" with "This muxer". The "segmenter muxer"
was never defined before, and thus it is a slightly ambiguous term.

doc/muxers.texi

@@ -676,7 +676,7 @@ overhead.
 
 Basic stream segmenter.
 
-The segmenter muxer outputs streams to a number of separate files of nearly
+This muxer outputs streams to a number of separate files of nearly
 fixed duration. Output filename pattern can be set in a fashion similar to
 @ref{image2}.