[flang][docs] Correct the title in Unsigned.md (#171553)

Probablly the original author copied `Extensions.md` and forgot to
change the title.

Author: kawashima-fj

flang/docs/Unsigned.md

@@ -6,7 +6,7 @@
   
 -->
 
-# Fortran Extensions supported by Flang
+# Flang support for UNSIGNED type
 
 ```{contents}
 ---