Fix docs build issue introduced by r363035

Replacing '.. code-block:: llvm' by '::' is a quick fix to the
build warning/error: Could not lex literal_block as "llvm".

llvm-svn: 363079

Author: sdesmalen-arm

llvm/docs/LangRef.rst

@@ -13766,7 +13766,7 @@ The second argument must be a vector of floating-point values.
 Examples:
 """""""""
 
-.. code-block:: llvm
+::
 
       %unord = call reassoc float @llvm.experimental.vector.reduce.v2.fadd.f32.v4f32(float 0.0, <4 x float> %input) ; unordered reduction
       %ord = call float @llvm.experimental.vector.reduce.v2.fadd.f32.v4f32(float %start_value, <4 x float> %input) ; ordered reduction
@@ -13827,7 +13827,7 @@ The second argument must be a vector of floating-point values.
 Examples:
 """""""""
 
-.. code-block:: llvm
+::
 
       %unord = call reassoc float @llvm.experimental.vector.reduce.v2.fmul.f32.v4f32(float 1.0, <4 x float> %input) ; unordered reduction
       %ord = call float @llvm.experimental.vector.reduce.v2.fmul.f32.v4f32(float %start_value, <4 x float> %input) ; ordered reduction