style: Update HOMEPAGE_URL in CMakeLists.txt (#44)

The value for the key of HOMEPAGE_URL is url-string type.

```cmake
project(<PROJECT-NAME>
        [VERSION <major>[.<minor>[.<patch>[.<tweak>]]]]
        [DESCRIPTION <project-description-string>]
        [HOMEPAGE_URL <url-string>]
        [LANGUAGES <language-name>...])
```

CMakeLists.txt

@@ -25,7 +25,7 @@ project (
 
   # Some informational package metadata. Not necessary for the build.
   # Sets PROJECT_HOMEPAGE_URL and PROJECT_DESCRIPTION
-  HOMEPAGE_URL https://github.com/herzbube/libsgfcplusplus
+  HOMEPAGE_URL "https://github.com/herzbube/libsgfcplusplus"
   DESCRIPTION "A C++ library that uses SGFC to read and write SGF (Smart Game Format) data."
 )