Changed most "backticks" (grave accents) into apostrophes.

Quotations that are embraced by tick marks now look better, in most fonts.

src/od65/main.c

@@ -222,7 +222,7 @@ static void DumpFile (const char* Name)
     /* Try to open the file */
     FILE* F = fopen (Name, "rb");
     if (F == 0) {
-        Error ("Cannot open `%s': %s", Name, strerror (errno));
+        Error ("Cannot open '%s': %s", Name, strerror (errno));
     }
 
     /* Read the magic word */