Fixed parameter name in docstring of RCA.fit() (#64)

Author: Callidior

metric_learn/rca.py

@@ -97,7 +97,7 @@ def fit(self, data, chunks):
 
     Parameters
     ----------
-    X : (n x d) data matrix
+    data : (n x d) data matrix
         Each row corresponds to a single instance
     chunks : (n,) array of ints
         When ``chunks[i] == -1``, point i doesn't belong to any chunklet.