Triggering CD4026 Reset w/ a CD4017
5 and 6 are unique in being the only two outputs where the \$b\$ segment is not lit. The two numbers can then be differentiated based on the \$e\$ segment.

If you have an inverter, then \$\overline{b}\cdot e\$ will tell you when 6 is lit, which can then trigger the reset. 

You can also include the \$d\$ segment as an input to your 3-input AND gate for extra resilience against slight delays in the output (prevent glitches when it counts from 4 to 5).