Fix typo in Arg Javadoc (a object → an object)

[hydrationn]

Summary

This PR fixes a minor typo in the Javadoc of org.apache.ibatis.annotations.Arg.

Before

• Return the statement id for retrieving a object that map to this argument. (line 78)
• Returns the result map id for mapping to a object that map to this argument. (line 85)

After

• Return the statement id for retrieving an object that map to this argument.
• Returns the result map id for mapping to an object that map to this argument.

No behavioral changes, only Javadoc text is updated.

This is a documentation-only change.

[coveralls]

coverage: 87.382%. remained the same
when pulling a4e6464 on hydrationn:fix-arg-javadoc-typo
into b814cba on mybatis:master.