DOC: Update wrong example in df.first doc-string

Author: Younggun Kim <iam@younggun.kim>

Closes #12310 from scari/master and squashes the following commits:

16acbcf [Younggun Kim] DOC: Update wrong example in df.first doc-string

Author: scari

pandas/core/generic.py

@@ -3939,7 +3939,7 @@ def first(self, offset):
 
         Examples
         --------
-        ts.last('10D') -> First 10 days
+        ts.first('10D') -> First 10 days
 
         Returns
         -------