Update fallback font from asciidoctor-pdf

This ensures that the u000a glyph is included. Using the current version
directly triggers another missing glyph warning which I fixed by generating
a new font version that has the heavy ballot box X character.
Update the existing font to use the new one from the asciidoctor-pdf pull
request asciidoctor/asciidoctor-pdf#2634.

Fixes: #187

Signed-off-by: Alex Richardson <alexrichardson@google.com>

Author: arichardson

fonts/mplus-1p-regular-fallback.ttf

@@ -29,11 +29,10 @@ font:
       bold: mplus-1mn-bold.ttf
       italic: mplus-1mn-light.ttf
       bold_italic: mplus-1mn-medium.ttf
-    M+ 1p Fallback:
-      normal: mplus-1p-regular-fallback.ttf
-      bold: mplus-1p-regular-fallback.ttf
-      italic: mplus-1p-regular-fallback.ttf
-      bold_italic: mplus-1p-regular-fallback.ttf
+    # M+ 1p supports Latin, Latin-1 Supplement, Latin Extended, Greek, Cyrillic, Vietnamese, Japanese & an assortment of symbols
+    # It also provides arrows for ->, <-, => and <= replacements in case these glyphs are missing from font
+    # TODO: use asciidoctor-pdf version (GEM_FONTS_DIR) once PR XXX is merged and used by the RISC-V docker container.
+    M+ 1p Fallback: mplus1p-regular-fallback.ttf
     Droid Fallback:
       normal: droid-sans-fallback.ttf
       italic: droid-sans-fallback.ttf
@@ -44,8 +43,6 @@ font:
       italic: JetBrainsMono-Italic.ttf
       bold: JetBrainsMono-Bold.ttf
       bold_italic: JetBrainsMono-BoldItalic.ttf
-    # M+ 1p supports Latin, Latin-1 Supplement, Latin Extended, Greek, Cyrillic, Vietnamese, Japanese & an assortment of symbols
-    # It also provides arrows for ->, <-, => and <= replacements in case these glyphs are missing from font
   fallbacks:
     - M+ 1p Fallback
     - Droid Fallback