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

include/mruby/compile.h

@@ -55,8 +55,8 @@ enum mrb_lex_state_enum {
   EXPR_CMDARG,                /* newline significant, +/- is an operator. */
   EXPR_MID,                   /* newline significant, +/- is an operator. */
   EXPR_FNAME,                 /* ignore newline, no reserved words. */
-  EXPR_DOT,                   /* right after `.' or `::', no reserved words. */
-  EXPR_CLASS,                 /* immediate after `class', no here document. */
+  EXPR_DOT,                   /* right after '.' or '::', no reserved words. */
+  EXPR_CLASS,                 /* immediate after 'class', no here document. */
   EXPR_VALUE,                 /* alike EXPR_BEG but label is disallowed. */
   EXPR_MAX_STATE
 };

include/mruby/opcode.h

@@ -8,7 +8,7 @@
 #define MRUBY_OPCODE_H
 
 #define MAXARG_Bx        (0xffff)
-#define MAXARG_sBx       (MAXARG_Bx>>1)         /* `sBx' is signed */
+#define MAXARG_sBx       (MAXARG_Bx>>1)         /* 'sBx' is signed */
 
 /* instructions: packed 32 bit      */
 /* -------------------------------  */