DOC: fix subject-verb agreement in merging.rst

[dinesh-git17]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.
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Description

Fixes a subject-verb agreement error in the merge_asof section of the merging documentation.

Current:

"For each row in the left DataFrame, the last row in the right DataFrame are selected..."

Fixed:

"For each row in the left DataFrame, the last row in the right DataFrame is selected..."

The subject "the last row" is singular and requires the singular verb "is" rather than "are".

[mroeschke]

Thanks @dinesh-git17