Remove unnecessary backticks.

Dr Markus Kuhn published in 1999 an article [1] explaining in details
why we shouldn't use the ASCII grave accent (0x60) as a left quotation.

Backticks have been used most notably to produce nice-looking LaTeX
documents but it doesn't seem to be an issue on modern platforms and
for the oldest ones, there are workarounds as mentioned by Dr Kuhn.

[1]: https://www.cl.cam.ac.uk/~mgk25/ucs/quotes.html

src/string.c

@@ -1100,7 +1100,7 @@ mrb_str_downcase_bang(mrb_state *mrb, mrb_value str)
  *
  *  Returns a copy of <i>str</i> with all uppercase letters replaced with their
  *  lowercase counterparts. The operation is locale insensitive---only
- *  characters ``A'' to ``Z'' are affected.
+ *  characters 'A' to 'Z' are affected.
  *
  *     "hEllO".downcase   #=> "hello"
  */
@@ -1703,7 +1703,7 @@ mrb_str_rindex_m(mrb_state *mrb, mrb_value str)
  *
  *  If <i>pattern</i> is omitted, the value of <code>$;</code> is used.  If
  *  <code>$;</code> is <code>nil</code> (which is the default), <i>str</i> is
- *  split on whitespace as if ` ' were specified.
+ *  split on whitespace as if ' ' were specified.
  *
  *  If the <i>limit</i> parameter is omitted, trailing null fields are
  *  suppressed. If <i>limit</i> is a positive number, at most that number of
@@ -2211,7 +2211,7 @@ mrb_str_upcase_bang(mrb_state *mrb, mrb_value str)
  *
  *  Returns a copy of <i>str</i> with all lowercase letters replaced with their
  *  uppercase counterparts. The operation is locale insensitive---only
- *  characters ``a'' to ``z'' are affected.
+ *  characters 'a' to 'z' are affected.
  *
  *     "hEllO".upcase   #=> "HELLO"
  */