[Flang][Doc] Fix trivial issues in HighLevelFIR.md (#72278)

Fixes section depth and removes annotation of source as HLFIR since this
is not currently understood by the lexer. Fixing these issues help
reduce warnings in the documentation CI for LLVM.

Author: kiranchandramohan

flang/docs/HighLevelFIR.md

@@ -41,7 +41,7 @@ The core impact on lowering will be:
     relevant.
 
 
-## Variable and Expression value concepts in HLFIR
+# Variable and Expression value concepts in HLFIR
 
 ## Strengthening the variable concept
 
@@ -1035,7 +1035,7 @@ HLFIR array assignment lowering pass:
     associated variables that are neither target nor pointers.
 -   Lower to assignment to loop:
 
-```HFLIR
+```
 func.func @_QPfoo(%arg0: !fir.box<!fir.array<?xf32>>, %arg1: !fir.box<!fir.array<?xf32>>) {
   %a = hlfir.declare %arg0 {fir.def = "_QPfooEa"} : !fir.box<!fir.array<?xf32>>, !fir.box<!fir.array<?xf32>>
   %b = hlfir.declare %arg1 {fir.def = "_QPfooEb"} : !fir.box<!fir.array<?xf32>>, !fir.box<!fir.array<?xf32>>
@@ -1320,7 +1320,7 @@ Lowering of vector subscripted entities would happen as follow:
   input IO) by inlining the elemental body into the created loops, and
   identifying the hlfir.designate producing the result.
 
-```HFLFIR
+```
 func.func @_QPfoo(%arg0: !fir.ref<!fir.array<?xf32>>, %arg1: !fir.ref<!fir.array<?xf32>>, %arg2: !fir.box<<!fir.array<?xi32>>) {
   %a = hlfir.declare %arg0 {fir.def = "_QPfooEa"} : !fir.box<!fir.array<?xf32>>, !fir.ref<!fir.array<?xf32>>
   %b = hlfir.declare %arg1 {fir.def = "_QPfooEb"} : !fir.box<!fir.array<?xf32>>, !fir.ref<!fir.array<?xf32>>