[rtext] fix: misuse of cast in GetCodepointCount (#4741)

I was really wondering what is going on here :D I believe this code
tried initially to out-cast 'const' specifier but this is not needed here
at all. Currently, it just confuses whoever reads this so I changed it.
The old code would also trigger -Wcast-qual warning on some compilers.

src/rtext.c

@@ -1915,7 +1915,7 @@ void UnloadCodepoints(int *codepoints)
 int GetCodepointCount(const char *text)
 {
     unsigned int length = 0;
-    char *ptr = (char *)&text[0];
+    const char *ptr = text;
 
     while (*ptr != '\0')
     {