Removed unpaired left-paren.  Rephrased references.
Merge pull request #7959 from mcjcode/docfix

DOC: Removed unpaired right-paren.  Rephrased references.
DOC: cookbook.rst update
ENH: Use 'left' constructor on pd.merge

Use the _constructor property when creating the merge result to
preserve the output type. Update test and docs appropriately.