Update Corpora_and_Vector_Spaces.ipynb (#772)

Change "Market Matrix" to "Matrix Market": typo.

docs/notebooks/Corpora_and_Vector_Spaces.ipynb

@@ -418,7 +418,7 @@
     "\n",
     "There exist several file formats for serializing a Vector Space corpus (~sequence of vectors) to disk. *Gensim* implements them via the *streaming corpus interface* mentioned earlier: documents are read from (resp. stored to) disk in a lazy fashion, one document at a time, without the whole corpus being read into main memory at once.\n",
     "\n",
-    "One of the more notable file formats is the [Market Matrix format](http://math.nist.gov/MatrixMarket/formats.html). To save a corpus in the Matrix Market format:"
+    "One of the more notable file formats is the [Matrix Market format](http://math.nist.gov/MatrixMarket/formats.html). To save a corpus in the Matrix Market format:"
    ]
   },
   {