UseJava: fix typo in variable name (#13135)

As Dave Abrahams pointed out CMAKE_CURRENT_SOURCE_PATH is wrong, it's of
course CMAKE_CURRENT_SOURCE_DIR.

Also wrap the path in quotes so the example would even work if the source path
has spaces.

Modules/UseJava.cmake

@@ -130,7 +130,7 @@
 #   Example:
 #   create_javadoc(my_example_doc
 #     PACKAGES com.exmaple.foo com.example.bar
-#     SOURCEPATH ${CMAKE_CURRENT_SOURCE_PATH}
+#     SOURCEPATH "${CMAKE_CURRENT_SOURCE_DIR}"
 #     CLASSPATH ${CMAKE_JAVA_INCLUDE_PATH}
 #     WINDOWTITLE "My example"
 #     DOCTITLE "<h1>My example</h1>"