Fix typo

Author: tblah

flang/include/flang/Parser/parse-tree.h

@@ -4894,7 +4894,7 @@ struct OpenMPSectionsConstruct {
   // Each of the OpenMPConstructs in the list below contains an
   // OpenMPSectionConstruct. This is guaranteed by the parser.
   // The end sections directive is optional here because it is difficult to
-  // generate helpful error messages for a missing end directive wihtin the
+  // generate helpful error messages for a missing end directive within the
   // parser. Semantics will generate an error if this is absent.
   std::tuple<OmpBeginSectionsDirective, std::list<OpenMPConstruct>,
       std::optional<OmpEndSectionsDirective>>