[SelectionDAG] Remove single quote around GET_ROUNDING in doxygen comment in ISDOPcode.h. NFC (#172114)

This style isn't used elsewhere in this file.

Hoping this fixes the strange rendering of the enum here.
https://llvm.org/doxygen/namespacellvm_1_1ISD.html#a22ea9cec080dd5f4f47ba234c2f59110

Author: topperc

llvm/include/llvm/CodeGen/ISDOpcodes.h

@@ -959,7 +959,7 @@ enum NodeType {
 
   /// Set rounding mode.
   /// The first operand is a chain pointer. The second specifies the required
-  /// rounding mode, encoded in the same way as used in '``GET_ROUNDING``'.
+  /// rounding mode, encoded in the same way as used in GET_ROUNDING.
   SET_ROUNDING,
 
   /// X = FP_EXTEND(Y) - Extend a smaller FP type into a larger FP type.