ICU-22774 Fix URL typo in error message

Author: echeran

icu4c/source/common/putil.cpp

@@ -76,7 +76,7 @@
 #include <float.h>
 
 #ifndef U_COMMON_IMPLEMENTATION
-#error U_COMMON_IMPLEMENTATION not set - must be set for all ICU source files in common/ - see https://unicode-org.github.io/icu/userguide/howtouseicu
+#error U_COMMON_IMPLEMENTATION not set - must be set for all ICU source files in common/ - see https://unicode-org.github.io/icu/userguide/icu/howtouseicu.html
 #endif