Fix typo in reading question in sec_seq-polynomial.ptx

In the reading question, the example given for a quadratic formula is 3,5,8,12,18.  The last term should be 17 if this is to be quadratic.

Author: neldredge-unco

source/sec_seq-polynomial.ptx

@@ -931,7 +931,7 @@ print(x)
       <premise>
       <m>3,7,11,15,19,\ldots</m></premise> <response>Linear formula</response> </match> <match order="4">
       <premise>
-      <m>3,5,8,12,18,\ldots</m></premise> <response>Quadratic formula</response> </match> <match order="2">
+      <m>3,5,8,12,17,\ldots</m></premise> <response>Quadratic formula</response> </match> <match order="2">
       <premise>
       <m>3,4,7,13,23,\ldots</m></premise> <response>Cubic formula</response> </match> <match order="3">
       <premise>