Replace 'FontName' with 'Fontname' in the documentation.

This is the only use of 'FontName' with that capitalization, as both
source-code and tests use 'Fontname'. Having consistent capitalization
makes it easier to find the relevant source from the docs.

See these examples for other uses:
    libavcodec/ass_split.c:68
    tests/ref/fate/sub-cc:9

doc/filters.texi

@@ -17988,7 +17988,7 @@ subtitles=video.mkv:si=1
 To make the subtitles stream from @file{sub.srt} appear in 80% transparent blue
 @code{DejaVu Serif}, use:
 @example
-subtitles=sub.srt:force_style='FontName=DejaVu Serif,PrimaryColour=&HCCFF0000'
+subtitles=sub.srt:force_style='Fontname=DejaVu Serif,PrimaryColour=&HCCFF0000'
 @end example
 
 @section super2xsai