I heard on the news that corn...

Whoever told you that corn contributes to humidity in the summer — is correct! Corn, like any other growing plant, releases moisture as a natural part of growth and life (transpiration). This release of moisture helps cool the plant. So, indeed, corn is releasing moisture all during its growing season, and hence is contributing moisture to the local moisture field — which results in increased humidity, both in the field (below the corn “canopy”) and in the surrounding area when this moisture is moved by local wind systems.