[LangRef] Fix missing code highlighting format

Author: aqjune

llvm/docs/LangRef.rst

@@ -3827,7 +3827,7 @@ cleared low bit. However, in the ``%C`` example, the optimizer is
 allowed to assume that the '``undef``' operand could be the same as
 ``%Y``, allowing the whole '``select``' to be eliminated.
 
-.. code-block:: text
+.. code-block:: llvm
 
       %A = xor undef, undef
 
@@ -3902,7 +3902,7 @@ predicates, such as Correlated Value Propagation and Global Value Numbering.
 In case of switch instruction, the branch condition should be frozen, otherwise
 it is undefined behavior.
 
-.. code-block:: text
+.. code-block:: llvm
 
     Unsafe:
       br undef, BB1, BB2 ; UB