Fix project reference path in documentation

Updated project reference path in preprocessor directives documentation.

Author: IEvangelist

docs/csharp/language-reference/preprocessor-directives.md

@@ -115,7 +115,7 @@ The `#:` directives that are used in file-based apps include:
   Instances of `#:project` are translated into `ProjectReference` elements to include the project with the specified path to the project. For example:
 
   ```csharp
-  #:project ../Path/To.Example.csproj
+  #:project ../Path/To.Example
   ```
 
   The preceding preprocessor token is translated into: