ICU-22465 Change .s to .data() for replacing TinyString with CharString.

This was forgotten by ICU-7496 which replaced the local TinyString data
type with the shared CharString data type, but as it's in code heavily
nested in #ifdef's it hasn't been noticed until now.

icu4c/source/common/udata.cpp

@@ -1196,7 +1196,7 @@ doOpenChoice(const char *path, const char *type, const char *name,
  *p = U_FILE_SEP_CHAR;
  }
 #if defined (UDATA_DEBUG)
- fprintf(stderr, "Changed path from [%s] to [%s]\n", path, altSepPath.s);
+ fprintf(stderr, "Changed path from [%s] to [%s]\n", path, altSepPath.data());
 #endif
  path = altSepPath.data();
  }