cmNinjaTargetGenerator: Replace "their" with "there" in comment

Source/cmNinjaTargetGenerator.cxx

diff --git a/Source/cmNinjaTargetGenerator.cxx b/Source/cmNinjaTargetGenerator.cxx
index 57fc020..57657b1 100644
--- a/Source/cmNinjaTargetGenerator.cxx
+++ b/Source/cmNinjaTargetGenerator.cxx
@@ -1749,7 +1749,7 @@
   } else {
     cmGlobalNinjaGenerator* gg = this->GetGlobalGenerator();
     std::string fullPath = gg->GetCMakeInstance()->GetHomeOutputDirectory();
-    // Also ensures their is a trailing slash.
+    // Also ensures there is a trailing slash.
     gg->StripNinjaOutputPathPrefixAsSuffix(fullPath);
     fullPath += path;
     cmSystemTools::MakeDirectory(fullPath);