Fix the build.

Author: kazutakahirata

llvm/docs/SourceLevelDebugging.rst

@@ -943,7 +943,7 @@ VirtRegRewriter pass re-inserts ``DBG_VALUE`` instructions in their original
 positions, translating virtual register references into their physical
 machine locations. To avoid encoding incorrect variable locations, in this
 pass any ``DBG_VALUE`` of a virtual register that is not live, is replaced by
-the undefined location. The LiveDebugVariables may insert redundant ``DBG_VALUE``s
+the undefined location. The LiveDebugVariables may insert redundant ``DBG_VALUE``'s
 because of virtual register rewriting. These will be subsequently removed by
 the RemoveRedundantDebugValues pass.