Fix copy-paste defect in comment.

Commit a7471bd introduced it.

Author: nmisch

src/include/c.h

@@ -101,7 +101,7 @@
  * GCC: https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
  * GCC: https://gcc.gnu.org/onlinedocs/gcc/Type-Attributes.html
  * Sunpro: https://docs.oracle.com/cd/E18659_01/html/821-1384/gjzke.html
- * XLC: https://www.ibm.com/support/knowledgecenter/SSGH2K_13.1.2/com.ibm.xlc131.aix.doc/language_ref/type_attrib.html
+ * XLC: https://www.ibm.com/support/knowledgecenter/SSGH2K_13.1.2/com.ibm.xlc131.aix.doc/language_ref/function_attributes.html
  * XLC: https://www.ibm.com/support/knowledgecenter/SSGH2K_13.1.2/com.ibm.xlc131.aix.doc/language_ref/type_attrib.html
  */