Update BotSharp.Plugin.KnowledgeBase.csproj

Downgrade `PdfPig` to v0.1.8.

Author: Oceania2018

src/Plugins/BotSharp.Plugin.KnowledgeBase/BotSharp.Plugin.KnowledgeBase.csproj

@@ -17,7 +17,7 @@
   </ItemGroup>
 
   <ItemGroup>
-    <PackageReference Include="PdfPig" Version="0.1.9-alpha-20240208-19734" />
+    <PackageReference Include="PdfPig" Version="0.1.8" />
     <PackageReference Include="TensorFlow.Keras" Version="0.15.0" />
   </ItemGroup>