doc/MANUAL.adoc: Don't use non-ASCII quotes (#761)

doc/MANUAL.adoc: Don't use non-ASCII quotes (#761)

Some locales like "LANG=fr_FR.iso885915@euro make" can't
handle UTF-8 single- and double-quotes:

$ LANG=fr_FR.iso885915@euro make
...
asciidoc: FAILED: MANUAL.adoc: line 529: unexpected error:
...
  File "/usr/lib/python3.8/encodings/iso8859_15.py", line 19, in encode
    return codecs.charmap_encode(input,self.errors,encoding_table)[0]
UnicodeEncodeError: 'charmap' codec can't encode character '\u201c'
  in position 54: character maps to <undefined>

To avoid it the patch uses ASCII equivalents of symbols.

The patch is generated as:

$ sed \
    -e 's/\xE2\x80\x99/'\''/g' \
    -e 's/\xE2\x80\x9C/'\`\`'/g' \
    -e 's/\xE2\x80\x9D/'\'\''/g' \
    -i doc/MANUAL.adoc

Reported-by: Christophe PEREZ
Bug: https://bugs.gentoo.org/762814
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>