update string.h.md; ref #2931

doc/api/mruby/string.h.md

@@ -69,7 +69,7 @@ Returns a concated string comprised of a Ruby string and a C string.
 Returns a concated string comprised of a Ruby string and a C string(A shorter alternative to mrb_str_cat).
 ### mrb_str_append
 ```C
-   mrb_value mrb_str_append(mrb_state *mrb, mrb_value str, mrb_value str2);
+   mrb_value mrb_str_append(mrb_state *mrb, mrb_value str1, mrb_value str2);
 ```
 Adds str2 to the end of str1.
 ### mrb_str_cmp

src/string.c

@@ -2385,10 +2385,10 @@ mrb_str_cat_str(mrb_state *mrb, mrb_value str, mrb_value str2)
 }
 
 MRB_API mrb_value
-mrb_str_append(mrb_state *mrb, mrb_value str, mrb_value str2)
+mrb_str_append(mrb_state *mrb, mrb_value str1, mrb_value str2)
 {
   str2 = mrb_str_to_str(mrb, str2);
-  return mrb_str_cat_str(mrb, str, str2);
+  return mrb_str_cat_str(mrb, str1, str2);
 }
 
 #define CHAR_ESC_LEN 13 /* sizeof(\x{ hex of 32bit unsigned int } \0) */