patch 8.2.3525: option variable name does not match option name

Problem:    Option variable name does not match option name. (Christ van
            Willigen)
Solution:   Rename the variable.
diff --git a/src/buffer.c b/src/buffer.c
index ba20a61..a3f84c7 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -2364,7 +2364,7 @@
 #ifdef FEAT_COMPL_FUNC
     clear_string_option(&buf->b_p_cfu);
     clear_string_option(&buf->b_p_ofu);
-    clear_string_option(&buf->b_p_thsfu);
+    clear_string_option(&buf->b_p_tsrfu);
 #endif
 #ifdef FEAT_QUICKFIX
     clear_string_option(&buf->b_p_gp);