riscv64: fix the name of Zisslpcfi.

Love these risc-v extension names!

Test: N/A
Change-Id: Ieb13f82a507aa11d79650b2cf627e68f213d6a79
diff --git a/libc/bionic/pthread_internal.h b/libc/bionic/pthread_internal.h
index a3a4ccd..106f3ed 100644
--- a/libc/bionic/pthread_internal.h
+++ b/libc/bionic/pthread_internal.h
@@ -137,7 +137,7 @@
   //    clobber x18 on arm64, therefore each process must declare early during
   //    process startup whether it might load legacy code.
   //    TODO: riscv64 has no legacy code, so we can actually go this route
-  //    there, but hopefully we'll actually get the Zsslpcfi extension instead.
+  //    there, but hopefully we'll actually get the Zisslpcfi extension instead.
   // 2) Mark the guard region as such using prctl(PR_SET_VMA_ANON_NAME) and
   //    discover its address by reading /proc/self/maps. One issue with this is
   //    that reading /proc/self/maps can race with allocations, so we may need