[docs] Fix missing quotes to preserve whitespace.
@@ -983,7 +983,7 @@ IMAGE_PATH =
# need to set EXTENSION_MAPPING for the extension otherwise the files are not
# properly processed by doxygen.
-INPUT_FILTER = python ../compiler_gym/third_party/proto2cpp.py
+INPUT_FILTER = "python ../compiler_gym/third_party/proto2cpp.py"
# The FILTER_PATTERNS tag can be used to specify filters on a per file pattern
# basis. Doxygen will compare the file name with each pattern and apply the
