[Doc] Added md5sum info for OGB-LSC dataset (#3332)

* Added md5sum for the large dataset files

md5sum helps in validating the correctness of large dataset files once downloaded. 

Refer: https://github.com/snap-stanford/ogb/issues/253

examples/pytorch/ogb_lsc/README.md

@@ -5,3 +5,11 @@
 - [Node Classification with MAG240M](https://dgl-data.s3-accelerate.amazonaws.com/dataset/OGB-LSC/mag240m_kddcup2021.zip)
 - [Link Prediction with WikiKG90M](https://dgl-data.s3-accelerate.amazonaws.com/dataset/OGB-LSC/wikikg90m_kddcup2021.zip)
 - [Graph Classification with PCQM4M](https://dgl-data.s3-accelerate.amazonaws.com/dataset/OGB-LSC/pcqm4m_kddcup2021.zip)
+
+
+# checksum md5sum of the files
+mag240m_kddcup2021.zip     : ```bd61c9446f557fbe4430d9a7ce108b34```
+
+wikikg90m_kddcup2021.zip   : ```73d4f5dde29d78669330b4db4c12fc9c``` 
+
+pcqm4m_kddcup2021.zip.     : ```5144ebaa7c67d24da1a2acbe41f57f6a```