Update functions-overview.md (#4470)
Update functions-overview.md (#4470)

In the current version, the interface 'org.apache.pulsar.functions.api.Context' does not have method 'getName()'.
On the contrary, the interface has the method 'getFunctionName()'.
So i change 'context.getName()' to 'context.getFunctionName()'.
merlimat/pulsar,merlimat/pulsar,merlimat/pulsar,merlimat/pulsar,yahoo/pulsar,massakam/pulsar,yahoo/pulsar,merlimat/pulsar,massakam/pulsar,yahoo/pulsar,yahoo/pulsar,merlimat/pulsar,massakam/pulsar,massakam/pulsar,massakam/pulsar,yahoo/pulsar,yahoo/pulsar,massakam/pulsar