[NFC] Fix a comment in MachinePassManager.h

Fix "warning: '\returns' command used in a comment that is not attached
to a function or method declaration [-Wdocumentation]
1 warning generated."

Author: Yuanfang Chen

llvm/include/llvm/CodeGen/MachinePassManager.h

@@ -91,7 +91,7 @@ class MachineFunctionAnalysisManager : public AnalysisManager<MachineFunction> {
   ///
   /// This method never runs the analysis.
   ///
-  /// \returns null if there is no cached result.
+  /// returns null if there is no cached result.
   using Base::getCachedResult;
 
   // FIXME: Add LoopAnalysisManager or CGSCCAnalysisManager if needed.