You have the C and F backwards, the correction is below:
on :TEXT:!temp c :#: {
notice $nick $3 degrees Celsius into Fahrenheit is: $calc(1.8 $3 + 32) degrees
}
on :TEXT:!temp f :#: {
notice $nick $3 degrees Fahrenheit into Celsius is: $calc(5/9 $3 - 32) degrees
}