DOC: minor format fix (#14802)

Author: swuecho

doc/source/cookbook.rst

@@ -107,10 +107,8 @@ Splitting
    df = pd.DataFrame(
         {'AAA' : [4,5,6,7], 'BBB' : [10,20,30,40],'CCC' : [100,50,-30,-50]}); df
 
-   dflow = df[df.AAA <= 5]
-   dfhigh = df[df.AAA > 5]
-
-   dflow; dfhigh
+   dflow = df[df.AAA <= 5]; dflow
+   dfhigh = df[df.AAA > 5]; dfhigh
 
 Building Criteria
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