doc/filters: improve pseudocolor example

Signed-off-by: Paul B Mahol <onemda@gmail.com>

doc/filters.texi

@@ -11851,7 +11851,7 @@ All expressions default to "val".
 @item
 Change too high luma values to gradient:
 @example
-pseudocolor='if(between(val,ymax,amax),lerp(ymin,ymax,(val-ymax)/(amax-ymax)),-1):if(between(val,ymax,amax),lerp(umax,umin,(val-ymax)/(amax-ymax)),-1):if(between(val,ymax,amax),lerp(vmin,vmax,(val-ymax)/(amax-ymax)),-1):-1'
+pseudocolor="'if(between(val,ymax,amax),lerp(ymin,ymax,(val-ymax)/(amax-ymax)),-1):if(between(val,ymax,amax),lerp(umax,umin,(val-ymax)/(amax-ymax)),-1):if(between(val,ymax,amax),lerp(vmin,vmax,(val-ymax)/(amax-ymax)),-1):-1'"
 @end example
 @end itemize