[NFC] Add a missing '.' to a comment.

Author: DougGregor

lib/Sema/TypeCheckError.cpp

@@ -134,7 +134,7 @@ class AbstractFunction {
         fn = conversion->getSubExpr()->getValueProvidingExpr();
       } else if (auto conversion = dyn_cast<BindOptionalExpr>(fn)) {
         fn = conversion->getSubExpr()->getValueProvidingExpr();
-      // Look through optional injections
+      // Look through optional injections.
       } else if (auto injection = dyn_cast<InjectIntoOptionalExpr>(fn)) {
         fn = injection->getSubExpr()->getValueProvidingExpr();
       // Look through function conversions.