avfilter/avf_showspectrum: improve magma colors

libavfilter/avf_showspectrum.c

@@ -248,8 +248,8 @@ static const struct ColorTable {
     { 0.35,            85/256.,     (138-128)/256.,      (179-128)/256. },
     { 0.48,            96/256.,     (128-128)/256.,      (189-128)/256. },
     { 0.64,           128/256.,     (103-128)/256.,      (214-128)/256. },
-    { 0.78,           167/256.,      (85-128)/256.,      (174-128)/256. },
-    {    1,           205/256.,      (80-128)/256.,      (152-128)/256. }},
+    { 0.92,           205/256.,      (80-128)/256.,      (152-128)/256. },
+    {    1,                  1,                  0,                   0 }},
     [GREEN] = {
     {    0,                  0,                  0,                   0 },
     {  .75,                 .5,                  0,                 -.5 },