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

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

src/co65/main.c

@@ -112,7 +112,7 @@ static void CheckLabelName (const char* Label)
     }
 
     if (*L) {
-        Error ("Label name `%s' is invalid", Label);
+        Error ("Label name '%s' is invalid", Label);
     }
 }
 
@@ -123,7 +123,7 @@ static void CheckSegName (const char* Seg)
 {
     /* Print an error and abort if the name is not ok */
     if (!ValidSegName (Seg)) {
-        Error ("Segment name `%s' is invalid", Seg);
+        Error ("Segment name '%s' is invalid", Seg);
     }
 }
 
@@ -244,7 +244,7 @@ static void OptO65Model (const char* Opt attribute ((unused)), const char* Arg)
     /* Search for the model name */
     Model = FindModel (Arg);
     if (Model == O65_MODEL_INVALID) {
-        Error ("Unknown o65 model `%s'", Arg);
+        Error ("Unknown o65 model '%s'", Arg);
     }
 }
 
@@ -387,7 +387,7 @@ int main (int argc, char* argv [])
         } else {
             /* Filename. Check if we already had one */
             if (InputName) {
-                Error ("Don't know what to do with `%s'", Arg);
+                Error ("Don't know what to do with '%s'", Arg);
             } else {
                 InputName = Arg;
             }