fixes docstring in to_hdf() (pandas-dev#42775)

Author: kadatatlukishore

pandas/core/generic.py

@@ -2624,9 +2624,9 @@ def to_hdf(
             - 'a': append, an existing file is opened for reading and
               writing, and if the file does not exist it is created.
             - 'r+': similar to 'a', but the file must already exist.
-        complevel : {0-9}, optional
+        complevel : {0-9}, default None
             Specifies a compression level for data.
-            A value of 0 disables compression.
+            A value of 0 or None disables compression.
         complib : {'zlib', 'lzo', 'bzip2', 'blosc'}, default 'zlib'
             Specifies the compression library to be used.
             As of v0.20.2 these additional compressors for Blosc are supported